if you're shooting video- does it transfer data fast enough for your camera output, does it have enough capacity for your camera output
for stills- it the data transfer rate fast enough for your buffer writing rate, does it have enough capacity for what you want to do with it.
Personally, I shoot a class 6, 16 GB SD and it is more than enough for stills (14 MP images in a D90). Some prefer multiple smaller cards. I prefer to take things off the card more frequently and always have a bunch of empty space spare on the card. Most anything from Class 4 and 4 GB and up is fine.
Buy from a reputable dealer and if you're concerned stick to name brands.
thanks
rick