It turns out that I'm a classic Explorer -- my enjoyment of a game comes from playing with different systems to see how they work, traveling to different places to see what they look like, and so on. So my problem with most games is that they completely marginalize this approach to gameplay, choosing instead a design ("classes") that strictly limits what you can what you can do according to your class and level.
Maybe it's just me, but I find that kind of thing stifling. I'm familiar with the ancient argument: "but there have to be classes so that players know their role in combat groups." So who says every game's character progression system has to be all about combat?
I like the idea of being able to pick up lots of different abilities from different fields. I know that means I'll never be able to enjoy the "uber" content that's constantly being developed for the dedicated level-grinders, but I'm OK with that because as a generalist, I get to see a lot more different types of gameplay than a specialist.
But that only happens if the game's design allows me to pick and choose abilities. The conventional MMORPG "pick a class, any class... and STAY THERE" design makes that impossible.
It's tempting to claim that Star Trek Online will be a better or more successful game if it allows players to gain abilities from different professional tracks. But I don't know that. All I can say is that I'd be a lot more likely to play that game than one that stuffs me into a class box and chains the lid shut.
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
--Robert A. Heinlein, Time Enough for Love