You can play any way you want to and do not have to jungle. However, some champs are so good at jungling, that one usually prefers that to normal laning (Amumu, Shaco, Skarner and the like).
You are right, that a jungler means that one other player is left alone against two enemies. However, he has the protection of a turret, which is enough in the early game to survive. Plus, he levels almost twice as fast as his two enemies, so he will very soon have his ultimate available against two level 3 or 4 enemies. Of course, some champions have a hard time to stay alone in a side-lane. Make sure when you have a jungler in your team, that you have a solo-champ that can hold its own without problems.
Jungling can only theoretically be done by all champions (sure, anyone can go into the jungle and attack creeps there). However, most champions are way too slow to make it worthwhile. They would profit much more from staying in a lane. These champions should not try jungling. If you want to try jungling yourself, then select a champion, for which you have one of these jungling guides at hand. Don't just try any random champion out in the jungle.
Finally, the most important point about jungling is one that most novice players often forget. You do not jungle in order to remain in peace and level up on your own. You jungle, so you can level up and at the right moment appear in a lane for a surprise gank. Enemies have to be constantly aware that a jungler may pop out of a nearby brush all of a sudden turning their 1v1 or 2v2 lane into a 2v1 or 3v2 problem. If done right, jungling can very quickly devastate your enemies, because the jungler has the power to turn around any lane. If one of your team's lanes is having trouble, adding another champion to it will devastate the enemies and at least break their attacks. Often, you can even net a kill or two. If you're a good jungler and help out frequently with ganks, then you will soon outlevel your enemies and the game can be pretty much decided in less than 20min.
So in summary, do not doubt jungling. It is a very popular and strong strategy. However, be aware that being a good jungler is not easy. Especially, when playing together with other novices, who may not know how to deal with a jungler in their own team. I suggest, you try to get at least several dozen games of normal lane-gameplay done, before you try out jungling yourself. And then, start with practice games, so you are certain, that you will survive the early jungle farming. It is most embarassing to die to the golem when no enemy was ever present.
So unlike League of Legends, Heroes of Newerth wasn't just designed to feel like DotA, but to actually be DotA. From Heroes, to items, right down to the default map, it is very much a DotA clone
... and S2 was really OK with this. So if you're looking for a gameplay difference, then they're few and far between.
- There are 22 "new" Heroes, many of whom share only an ability or two with DotA heroes
- The map is smaller/faster, depending on your perspective, the map is either smaller or things move faster
- Players can rejoin games they disconnect from, if you've ever played DotA you know why this is gameplay related
- 8 Unique Items
- Some orb effects stack
- Move modifiers stack via multiplication
- Players can forfeit during a game
- All abilities are bound to qwer
- The HUD is constantly focus on your Hero with the option to examine other Heroes
- Shops can be used at all times via the b and are sorted for easier use
- You can even buy parts of a recipe from that recipe
I admit this list is a little strapped for items, but when you design a game to be the same... don't be surprised when its the same.
On the other hand, if you're willing to look outside of gameplay, the major advancement HoN has over DotA is a ranking system and matchmaking. For those people who didn't resort to third part products, you are probably familiar with how frustrating finding the game type you want is with DotA or finding people with equal skill to play with. HoN solved both of these problems exceptionally, using an Elo system to rate and allowing the searching of games types and restricting certain games by rating.
Finally, HoN allowed for the creation of new maps to aid in different game play style (one bizarre example is a map that has everything cleared out of it, and only 1 lane).
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Some of the most noticeable changes that pertain to League of Legends:
I'll add more as I think of more. Hope this is a good start for you!