Things I look for in a hack? It depends on the game of course, but in essence...
1-- As many have already stated: Game Improvements. Game developers usually do a good job with the work they put in, have the game play tested, etc...but sometimes, things slip through the cracks. Whether because of time constraints, budgets, or just plain oversights, sometimes things don't get done properly, and some portion of a game is either weak, not well implemented, or even broken. Some awesome rom hacker would go in, fix those things, maybe add some things the original designers never intended, but still work well with in a given game.
2-- Challenge for some games. Not talking Kaizou stuff (spelling?)....not ball bustingly, frustratingly, gotta-glitch-to-get-through it nonsense. Talking about a legit challenge added to a game. Something that makes the player really work for the win or to beat the game, but isn't impossible, and also still carries the basic fun of the game. It's a fine line, I know, and not many hackers truly understand that line, but hats off to those that do.
3-- All new adventures/levels/goals in games we know oh-so-well. Imagine my surprise when I first discovered rom hacks, and having played the original Legend of Zelda to death, saw that some people actually created ALL NEW adventures using the sounds and graphics of a game I was very familiar with! Can't say enough about how awesome old games turned into new ones are! I mean, even if the hacker doesn't go in and change the basic mechanics, but simply creates a brand new set of levels, worlds, etc, to explore that makes the same ol, same ol game feel new again, well, that is just priceless!
4-- Crossovers. Now, I don't get uber-excited about this one, but done right, it can work nicely and be fun too. Things like Mario themed Mega Man levels, or certain characters from one game put into another (again, if it's logical), or even 'skinned' type games where the original game and its gameplay is kept pretty much intact, but some other theme was skinned onto it...example, the Mario version of Balloon Fight.
5-- Finally, translations. Now, I know this site separates 'hacks' and 'translations', but technically, if you go into a rom and alter the font to another language, you kinda ARE hacking it as well. So, IMO, translations can fall under things I look for in rom hacks. Nothing like wanting to play a game for a long time but couldn't due to the language barrier, only to come across some fine hacker who went through the trouble of making it so I can understand and enjoy the game in my language. Tons of Japanese games I have enjoyed due to this!