No, you'll just earn 6 cards (1 legendary and 5 'commons').
Cards you'll earn :
Haunted Creeper good for a zoo deck (it's a deck with low cost cards and very agressive) you'll earn this if you beat Anub'Rekhan in normal.
Nerub'ar Weblord good for counter battlecry, you'll earn this if you beat Faerlina in normal.
Nerubian Egg it'll combo with a solfurie or an Argus, because if you destroy it, you'll earn a nerubian (it's a 4/4 cards), you'll earn this if you beat Maexxna in normal.
Maexxna she destroy any card she attacked, it can be usefull if you play Anduin Wrynn because he can heal Maexxna. You'll earn this if you beat all the 3 boss in normal and you'll unlock heroic and druid and rogue class challenges.
Poison Seeds : destroy all monsters on the battlefield and give treants 2/2 for each destroyed monster. Can combo with a druid token. Unlock when you finish the druid class challenge.
Anub'ar Ambusher a 5/5 for 4 mana but with a Deathrattle (Return a random friendly monster in your hand) can combo with Si 7. Unlock when you finish the rogue class challenge.
You can find many others information on the wiki : http://hearthstone.gamepedia.com/Curse_of_Naxxramas
The gimmick behind beating Loatheb is quite simple and a really nice twist to the usual way Hearthstone is played. It is all focus on you taking advantage of the spore card
The popular YouTuber Trump accidentally stumbled into a great deck against Loatheb in the form a control Shaman deck (also using only F2P cards). The basic idea is to maintain a strong board while keeping his clear, this means when the removal cards come out and the spores are down, you can take full advantage of the massive damage boost. Shaman is paticularly strong because of the ability to play a totem almost every turn, as well as thing like Feral spirits to get 2 minions from 1 card; putting more presence on the board.
Another good deck would be a Paladin; the ability to 'get a guy' each turn will be very strong when combined with the spores. Maybe even a divine shield paladin that can use early creatures such as the Argent Squire and Scarlet Crusader to build board presence while possibly soaking the spore creating AOE with the divine shield, leading to a strong next turn.
Update:
I saw a great deck that can make outstanding use out of the spores against heroic Loatheb. First of all, a big problem with heroic Loatheb is the fact he starts with a 3/6 Fen Creeper on turn 1! That in itself can be too much for many decks to handle right from the beginning. It could take several turns to take that thing down and by then he has probably played a few more strong minions, the game is over before you can even kill a single spore! However, a great way to counter this is to roll a Rogue deck. Saps will sort that Fen Creeper right out.
Rogue is also very effective for the following reasons:
Sap is essential for either getting rid of the initial Fen Creeper and getting rid of bigger minions.
Fan of Knives is a great mass spore killer and the extra card draw in this deck will be very useful.
Your ability to create a dagger is also very useful for killing the odd spore which means you don't have to run a minion into it or spend mana on removal, meaning you can push more damage through to his face.
SI:7 Agent is a great way of getting a good minion out and popping a spore at the same time, ideally you kill spores when you have multiple minions out but using an SI7 Agent on a spore is like playing an 11/3 minions for 3 mana.
General removal like Backstab and Eviscerate is very useful for board clear or killing off more spores.
The Deck List
The Essentials:
2x Sap : Almost required to remove the Fen Creeper early, it is also possible with Backstab+Eviscerate but thats an extra card you need to draw so it's less likely (but arguably more effective). You will have the highest chance of winning with a Sap in your opening hand.
2x Backstab + 2x Eviscerate : Remove is vital for clearing minions that need to go or popping spores.
2x Fan of Knives : Fan of Knives is awesome for taking out a pack of spores, also drawing another card that is very helpful in a deck like this.
Every 1/2 1 mana card + cheap charge minions + (Stonetusk Boar) + Wisps!: This is the main theme behind this deck. Flooding the board is very important for knocking down his health, combined with the spores you can pump out a ton of damage using cheap, usually worthless minions.
A possible version of this deck : Anti-Loatheb Rogue Deck
The deck is quite simple and should be really cheap if not free. The basic idea behind is is to use as many 1/2 minions as possible. Why? Well, simply because your minions damage is practically irrelevant! The only major damage you will be doing is through using the spores so it is much more important to go for quantity of minions instead of quality.
Why 1/2, why not 2/1? Because he has removal in the form of a 1 damage to all enemy minions, so if you fill your board with 2/1's they all die. The great thing about using the 1/2's is that that 1 damage aoe removal also spawns a spore. This means that on your next turn, ideally all of the cheap minions you flooded the board with will be alive and there will be 1 or more spores just waiting to give you a board full of 9/1's +.
Another thing to note, while you have no control over it, Loatheb is awful at using his Faceless Manipulators! If there is a spore on the board and he decides to play a Faceless Manipulator, chances are he will use it on the spore. I don't think I need to tell you how that is not a good idea for Loatheb. I even saw him use a Faceless on a spore when a Sylvanas Windrunner was also on the board! (she is not part of this deck).
Best Answer
The Adventures are usually very easy to not scare away casuals. To give experienced players a challenge as well they made the heroic mode. You fight the same bosses there as in the regular adventure, except they are a lot harder to beat.
While you can usually take your ladder decks to beat the normal adventure, you really need to fine-tune your decks for the heroic bosses.
To answer your question, the point of heroic is to offer experienced players a challenge. The only prize you get for beating all heroic bosses is a special card back.
Fun fact: Blizzard was actually thinking of doing that but they ended up not to, because they did not want to have too many things in the game, that casuals can not achieve.