Don't play table-top RPG's so my response is more to the videogamer crowd. I tend to avoid any class that has an overly-linear play style. I don't mind when my class does A well, as long as I have other options within the class just in case A isn't really working for me. For example, in Oblivion, if I make a combat oriented character I'm going to make sure that he's proficient in at least one school of magic.
Also, I tend to avoid classes which take up a large amount of inventory space. I prefer to only have to go through a dungeon once, and like to be able to go in, loot everything, and leave in a single trip. As a rule of thumb, I generally like to be able to carry 2 weapons, armour, and various other resources (food, healthpacks, ammo, repair kits, etc.) and still have at least half of my inventory available for loot, and classes based around heavy weapons/armour don't allow that. In Fallout 3, for example, I never used Big Guns because I hated having a quarter of my inventory space being taken up by a single weapon, and if it was not for the boost to strength I would never touch something as heavy as Power Armour.