I like the flavor that explicit classes bring to games' mythos/universe/story but I find myself always favoring the skill systems of games like Ultima Online with their flexibility and better-by-usage approach.
I think a nice marriage of these systems is something that begins as the UO system and has opportunities at a later stage of the game to opt-into a "class" system.
Something like completing a task to initiate yourself as a... Paladin? or something which grants specific abilities to your skill set that match the flavor of the job/class/archetype but maybe hinders other ones, based on flavor.
This way a base skill-setter isn't persuaded to ALWAYS opt-in to one since the restrictions maybe be seen as too big a negative.
I'm rambling a bit. Just wondered what others thought of class systems, ones they've liked, disliked, etc.