Is there any replay value in the Curse of Naxxramas solo adventure?
Solution 1:
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
Solution 2:
If you have the "Win 7 games in any mode" quest, Solo Adventure matches count, which includes Naxxramas.
So if you're just looking for a quick way to win against a known deck, bumrushing Maexxna with a charge deck is gonna be an easy way to get your 7 wins.
Beyond that, there is no reason to replay the wing.
Solution 3:
Currently, no. Once you earn all of the rewards for the quarter, there is no point in going back and replaying them.
I would assume that at some point in the future, people will do races to see who can go through Naxxramas the fastest or something along those lines.
Solution 4:
You can always try to challenge yourself with building different kinds of decks that will beat the bosses in heroic mode (or normal mode).
To start with, one deck of each class. If you manage that, you can try imposing other limitations on yourself (only rush, only control, etc.). That way you can make many different interesting challenges. Your imagination is the limit.