Putting aside for the moment some recent business contacts which are varyingly sketchy, GURPS has a few notable issues.
1) It loves its acronyms. This is more stylistic and less mechanical, but part of the reason it feels heavy is that GURPS seems to favor some sort of obtuse acronym instead of a descriptive word whenever given the choice.
2) Varying skill defaults are a pain. Skills are incredibly narrow, which is fine (narrower than I'd like, sure, but that's a preference), and they default to each other off varying penalties, which makes sense. In practice though this means that there are tons of tables floating around for whenever you use a skill you don't have, which is often, as they are numerous.
3) Combat. The combination of the 1 second round, active defenses, grid combat, and facing mechanics tends to produce something on the slow side.
4) Derived stats, and the use of tables. Damage is a great example here, but they're all over the place. This is especially true in certain optional subsystems, which brings me to...
5) GURPS Vehicles. It basically goes full Phoenix Command here, and even die hard GURPS fans tend to be lukewarm on it at best. That's pretty irrelevant though.
Basically, GURPS is in a space that I really like (fairly simulationist generic systems that support bizarre settings well), but there's enough clunk to its crunch that I tend to favor a simplified descendant. Usually Fudge, which was written by a GURPS author and keeps a lot of that ethos.