Have a fair # of 3rd ed. sourcebooks, as well as the core books, just sitting around on a shelf. My g/f likes the basic info contained in the sourcebooks (she's a history buff).
But, as nice & generic as it is, it really feels uber-detailed to me--a lot of detail that needs to be kept track of.
I'm not sure if there'll be as significant of a shift from 3rd to 4th (vs. D&D's shift from 2nd to 3rd)--rather, it may more or less be more comparable to D&D's shift from 3.0 to 3.5--esp. with issue regarding 3rd ed. sourcebook capatability with the core 4th ed. stuff. Honestly, if that's the case, I'm not too sure I'm give this newer version a try at all.
Then again, I do feel that "less = more," and don't care for a strong push toward capturing reality in games, since there's often a strong urge by some players to use that reality in order to help them break it (if you catch my drift).
Though I do love the polyhedral dice, just using d6's makes it a bit more accessible (to a degree)--heck, you'd just need to buy a brick of d6's, or at least raid a bunch of boardgame boxes &/or buy some pairs of dice in order to be equipped for the game. Keeps things somewhat simple, in that regard.
I also like the idea of point-buy for PCs, since it (generally) allows each character to start on even footing.
But, I think 1 of the big things about GURPS that gets to me is, from my experience, there seems to be a lot of hard-wired mechanical aspects/bonuses tied to things that could merely be covered via roleplaying, such as personality quirks & the like. Also it seems that some very liquid elements about characters--things like wealth, contacts, possessions, reputation, etc.--seem a bit more hard-wired into character creation as well. These things seem too easy to lose & gain for a PC, esp. through roleplay (if not, then things seem a bit too static for a PC, IMHO).
Also, I'm not a fan of uber-detailed combats. The more a combat is stretched out by additional levels/processes/details, the slower combat feels (& doesn't help to add to the feeling of a fast-based, urgent situation at all).
I understand that some may like to include things like this, &, of course, it's up to the GM to include/exclude things as he/she wishes. However, IMHO, I prefer that things merely be added into a game to include them for those who want them, rather than having to take things out to achieve the same thing.
Of course, that's just MHO on the matter. I think that, despite the revisions, GURPS (like D&D, WW WoD, and many other RPGs) will fit into a particular niche, and maybe expand a little, but I doubt that it'll bust out and take over other niches held by other games.