Well, a lot of it depends on what you're aiming for, honestly.

Although typically strong off-flavors will point to contamination: buttered popcorn, vinegar, or anything that shouldn't be in the finished product can point to Bad Things in your beer. As long as everything is kept reasonably aseptic from start to finish, you shouldn't have to worry.
Of course, certain yeast strains can add characteristics to the beer, but it shouldn't come as a surprise if that's the type of beer you set out to make. Example: many Belgian yeast strains will give those banana & clove esters, which are desirable in a Belgian beer (example: a Wit or an Abbey/Dubbel). And there's normal spice smells, such as those from bitter orange peel and coriander seeds.
Also, some beers can get a "cidery" taste if you add waaay too much sugar in an attempt to spike up the alcohol %. Typically when you follow the recipe, you'll be fine.
That's about all I know. I know that last year, my SO and I had to get rid of a plastic primary fermenting bucket, because it was scratched, so we couldn't sanitize it properly, and so we were getting that weird "buttered popcorn" contaminant flavor in the beer.
Kelli, I've never brewed a Belgian - how can you tell the difference between a good Belgian and a bad (contaminated) batch?