[Gudiomen]

Well, I suppose there's a bit of a learning curve, but there was one in 3e too. GURPS was actually the first system of RPG I purchased and played (except for a very questionable set of rules that me and a friend of mine came up with on our own when we heard about tabletop RPGs).
When 4e came up, the "mainstream" shouted in my face, and at first I was a little preocupied with it.
GURPS 4e Lite scared the begeezers out of me, and I still fail to understand how GURPS 3e Lite can be so much better in conveying the system's feel while 4e Lite simply gives you a vague abstract idea.
Anyway, the improvements were imediately obvious when I started reading. Things were streamlined (offcourse, I figured this being acostumed to the formulas in 3e, and with the conversion to metric system in portuguese you can imagine how the formulas look like), but there was definately more pop in it. Somethings, such as powers, got more complex, but I saw in a single book the ability to do anything, as oposed to having to buy a bunch of supplements.
Not having really used powers in 3e, or the enhancements and limitations, this part was the one that took more time to sink in, but it was also the key to what made GURPS better for me. Something as simple as Atractive (Takes Preapartion, 1h) is just an example.
Loosing half-points, using linear point progressions, using the same progression of points for all skills, etc. Makes some of the most tricky points of 3e actually easier.
I think the difficult stuff to learn for me was the new stuff and the sheer volume of stuff, but this is what makes the system universal. And while it did move a little more into mainstream, it didn't loose the realistic, grit options (not much anyway). On the whole, 4e was a whole new feel. And the 3e legacy will live forever in my brain, and I'll have a sense of low-level and grit stuff (wich I love) that I can still use in 4e. Offcourse, new players won't be so prone to it, and I worry that if everyone moves into mainstream the few of us by the shore will have no one to care for us, but so far GURPS still supports us quite well.
On the whole it's simply different. And I still have my 3e books to fall on, and am not shy to teak the system if I have too. And there's allways GULIVER for the people who really like math.