Comparing...

Unisystem/Witchcraft
- Point-based character creation (with categories) and advancement.
Pros: It's relatively simple to pick up with a few details that are more complicated. The rules move pretty quickly most of the time.
Cons: It feels incomplete - often someone will ask "okay, I can do this, so can I do this which is similar?" and there won't be an answer in the books. Some people get tripped up with the Rule of Ten/One, which is a critical/fumble system that requires a little calculation and/or the char. While specific campaigns might alter things slightly, the as-written balance between certain powers/types is somewhere between passable and horrid.
Other Thoughts: I don't know why some people I know are so fascinated with the system, to be honest. It's at its best when the PCs are a group of starting characters - slightly better than average Joe stats with some supernatural stuff in a mostly mundane world. However, if players min-max there or when they get more points, it rapidly becomes a game of "anything that can threaten Combat Monkey A can accidentally splatter Character B."

Exalted
- Point-based character creation (with categories) and advancement.
- Heavily manuever-based resolution system.
- High power to OMGWTF!
Pros: Though it involves a lot of dice, the basic system isn't that complex. The system reflects and fits the over-the-top setting.
Cons: It involves a lot of dice. While the basic system is simple, it's complicated immediately by Charms, which are a big part of the game and apply to almost every possible aspect of task resolution. The sheer volume of charms and possible interactions can be overwhelming. Conflict generally isn't going to be fast between to exalts of similar power.
Other Thoughts: Exalted will always hold a place near and dear to my heart, oddly. I like the high fantasy setting mixed with fantasy-flavored technological aspects. I like how enabling the game is, encouraging great things of the characters.
Unfortunately, you almost have to have the mindset of a tournament-level Magic player to make the most of the system. And combat rapidly goes from being awesomely over the top to ... well, a very strange situation where it becomes more a matter of bluffing and efficiency over an opponent because everyone has some supreme defense and an attack that will slaughter an opponent if it's not defended against. At that point, it tends to slow things down a lot from what I've seen. Also, I'd have to say the game gets a little over the top even for my wildest high fantasy dreams.

d20D&D:3E
- Class-based character creation, level-based advancement
Pros: Fairly simple to pick up. There can be a lot of modifiers, but they tend to be straightforward. d20 was meant to be pretty generic, and it manages to do so - for the most part. It's strongest in a D&D-type setting.
Cons: If you include supplements, it rapidly becomes unwieldy. As-written, it's designed more toward gain of XP by killing things rather than accomplishing goals. There are some distinct limitations in the class/level system.
Other Thoughts: Okay, I suppose I'm a bit of a d20 fanboy. I don't think it's the greatest thing since sliced bread, but I like it and can see a lot done with it. The core rules of the game are pretty generic, but not totally so.
Basically, I could do just about anything I wanted with it. But... I'm not sure whether to call that a virtue of the game system or just of my familiarity with it at the time. I has to admit that there is/was work involved in making suitable prestige classes or tweaking spell lists or whatnot to suit a setting.

GURPS
- Point based creation and advancement.
Pros: It's what it advertises in that it can be made to do just about anything.
Cons: I've rarely seen it do anything very well.
Other Thoughts: We played this for a while, and there were a ton of nit-picky rules. I don't know if they came from supplments or were homemade. Either way, the system always struck me as for those people who like to customize every little detail of everything - and I've usually felt that too much work.