Nope, that's a perfectly reasonable choice. Here're a couple of ways to build a net-and-spear character. (Which is a great archetype.)
Fighter
Choose fighter if you want to be more about controlling the battlefield than doing damage. I think the net works somewhat better here -- nets, like fighters, are all about control.
You'd probably want to take the tempest fighter build, which improves your two-weapon skills. Use a spear in your main hand, and the net as your offhand weapon. Consider the Net Training feat: with that, every time you hit someone with your net you'll slow them, which is a big part of what arena fighters traditionally get out of nets. You could also look at the Binding Style feat, which specifically allows you to immobilize a target when you hit it with your net and the Dual Strike power.
It's useful to have a high Dexterity, but in this context "high" means something more like 14 or 16; you don't need to get it up to 18.
I wouldn't, by the by, use the arena fighter build from the Dark Sun book itself. It's an interesting build but it's about improvised weapons; since you have specific weapons in mind it's not as good a fit.
Barbarian
Barbarians are about damage, and the net is not a very damaging weapon. Still, if that's the way you want to go, it'll work reasonably well. You'd want the whirling barbarian build, which is designed for barbarians with two weapons. Net Training is still a good feat, but not as important as it would be for the fighter, because it's a control-oriented feat rather than a damage-dealing feat.
General
Don't get hung up on the names of the classes. This is probably obvious, but if you want to play a fighter who comes from a barbaric tribe and thinks of himself as a barbarian, that's totally legitimate. D&D doesn't always make that as clear as I'd like, so I always take the chance to remind people.
The Gladiator theme is awesome for either the fighter or the barbarian. That goes without saying too, I know, I'm just being completist.
How To Introduce Limited Player Narration
I was introduced to this kind of play with Robin Laws' excellent Feng Shui RPG. It explicitly incorporates this kind of "light" player narration (as opposed to full co-authorship as is common in indie RPGs today). I still use it as training wheels to help people get into the mode of improvising. In Feng Shui, not only is use of tactically-correct battlemats contraindicated, but you're encouraged that if you are, for example, in a fight in a pizza parlor, to just say "I snatch up a pizza cutter from the table and cut the fool attacking me with it" without having to ask the GM or roll for the pizza cutter. If it makes sense and is genre appropriate it's fine.
Many people need that help to get started with the idea because most old school trad games definitely come from the paradigm that the player isn't allowed to contribute to the narrative at all - if the GM does not explicitly instantiate something, it doesn't exist. Which has some good attributes - too much player narration actually leaves me cold as a player since I prefer immersive character play - but some light player narration does make the game run more smoothly and be more exciting.
Just Say No!
In fact, in general I tell people they need to man up and improvise those kinds of things themselves because if they ask the GM I'll say no! "If you ask me how high the helicopter skid is, I'll say 'too high.' If you say 'I jump and grab the helicopter's skid,' I'll say 'roll it!' If you ask me if there's a pizza cutter within reach I'll say no..." That breaks them of the mother-may-I syndrome, which slows down the game. Having to say no once in a while is less time spent and more empowering.
Run a One-Shot To Introduce the Concept
With my current play group, I ran a one-shot of Feng Shui before launching into a longer campaign to train them on the mode of play. This gave them an opportunity to experiment with limited narration in a safe short mode environment. Then when we started a longer game in a traditional game system, the techniques port right over. You can use another game, just make sure it's yielding the type of narration you want - if you go too far into indie land then you'll get your normal D&D players saying "I declare the lich king is my long lost aunt and he's really after the strumpets on the moon!" (Fun fact: on Golarion the moon is full of strumpets, for real.) Which is fine if that's the kind of game you want, but that's not this kind of limited-narration environment which remains sim/immersion friendly while still getting a lot of the benefits of a narrative game.
Controlling the Narration
As for controlling the narration, we have very seldom had problems with that. We had the understanding that narration had to be very limited and appropriate - "belaying pin on the deck of a ship" is a valid pickup, "magical metal detector" is not - and the GM can always say no. Generally this is self policing, as "I come up from behind the pizza oven with a rocket launcher!" triggers everyone's cheese alarms and they just frown at the overexcited guy till he comes up with something less stupid (unless you're playing Toon or something very postmodern). All the "yes, but" stuff is fine for certain kinds of games, but it is unnecessary and disruptive for a game trying to be sim with limited narration only. A couple "no"s to set limits is much less hassle for everyone than trying to put conditions on inappropriate things. With adults it really won't come up much at all after some initial boundary exploration.
Mechanics and Limited Narration
Mechanics to dictate this are not needed and in fact I find them to be intrusive and harmful. The goal of limited narration is to be able to improv; if you have rules it's not really improv-ing. Whenever I have to shepherd currency (like FATE points) to do something cool, or make a roll separate from the roll I was going to make anyway to do a stunt (like the rolls for improvised aspects mentioned in the comments), that's lame because it 1) brings in more metagaming, taking away from character immersion and 2) takes away from the velocity of the game, which limited narration is intended to increase.
The one mechanical note I'll make is that people will try innovation to the extent that it's not punished - if you put big penalties on improvising it will tend to work against it. Feng Shui, as this is what it's about, went to the extent that you actually got a bonus to your rolls when doing a cool stunt. In d20 it's easy to rack up "-8 to do that because you don't have the 'slash a fool with a pizza cutter' feat" - make sure and encourage actions you want to encourage with the numbers. Removing a penalty or adding a bonus based on how cool something was is completely within your purview as a GM, despite the bizarre newthink from D&D 3e/4e that says it's not.
In D&D 2e, I had a halfway mechanic that worked - a Luck stat one would roll to see "if there's a pizza cutter". This worked in that campaign only because it was a very, very gritty low power sim game. I wouldn't use it in a normal power game and would just go to the "yep, if it's not dumb you've got it" model.
Though be careful of trying to over-mechanize it like Exalted does as described in @SimonGills' answer - I remember playing Roanoke, where you get a +1 per descriptive word you use on every action, up to +5. It led to some pretty sad results,as people just strung together tortured 5-adjective strings to get the bonus since the rules rewarded "5 words" instead of the desired result of "a cooler game." I prefer just "that's cool you get the +2 bonus" or not, when you start trying to over-define it it turns into a rule to lawyer instead of an improvisation to add to the story. Also, our stunt level is usually at their +4/+6 level - "I scream and hack at the orc" is expected in decent RP, not bonus worthy.
Best Answer
This is a system transition issue, not a creativity issue.
4e is a very different system and that's okay, but it's not for everyone. There's a gap between the player and the system and your job as GM is to help facilitate bridging that gap. Your goal in this should not be to make the player conform to the system, but to help the player understand the system's approach enough to decide if the system is one she wants to play in.
It's important to realize that creativity is not 'at stake' here: you can be just as creative in 4e as in any other system... just not in exactly the same fields.
What's the difference?
4e instituted multiple ground-level philosophy changes that build on each other:
Parties are hardy and self-contained. Instead of needing to exploit their environments and using dirty tricks to desperately survive on 3 hp and exceptionally limited healing, even the lowest-level 4e party has everything it needs to survive and triumph all written there on the character sheets.
It's not the net that's special: it's the person throwing it. 4e has feats, features, powers, and magic enchantments which could let a PC mimic that power's restraining effect. The net is probably the least important part of the equation; in 4e items are important and necessary but rarely definitive.
Realism takes a back seat to modular balance. Early editions left a great deal of mechanics to what the GM thought felt right, and 3.5 introduced numerous subsystems to provide rules for the same effect. This led to (often unexpected) problems with silly combinations and massive power imbalance. In order to reduce these problems, 4e abandoned most of its predecessors' attempts to mimic a 'real world' feel to its mechanics: it replaced them with standardized power gain, standardized effects, and a universal class system.
Together, means that rules are really important to 4e.
In earlier D&D editions, the player's line of reasoning would be absolutely correct: while a player has ability, most often it is the item the player wields which defines the effects the item can achieve. This was a major theme of D&D for a long time: the item makes the hero, and the heroes MUST grab every advantage they can or they will perish.
But in 4e, combat mechanics are balanced so carefully and already weighted so firmly in the players' favor that remaining within the game's mechanics is important to keep the game from spinning out of control. It's not your responsibility as a GM to make that happen invisibly; it's your responsibility to invite the players to work with you to make the game a good one.
To this end, player creativity in 4e needs to be channeled (voluntarily by the players, with support from the GM) away from scrounging and toward creative application of the existing rules. Finding how effective the nets are might make the PC want to start taking feats and features to learn how to use them, for example.
4e's fun is inside the box
4e is appealing to two major groups of players:
Other players may or may not find its modular philosophy chafing, but remember that any system can support strong, creative role-playing... just not always in the same areas. It's important for everyone at the table to be on the same page about the kind of system you're playing, or misunderstandings like your nets will crop up.