I have no knowledge of 4th ed, but I do own 1st-3rd. Some reasons I'll never run a GURPS game:
1 second rounds: Seriously, if you use an axe, half of your actions will be "ready weapon". How fun!
3d6 task resolution: This gives a bell shaped curve. As a result, a modifier of "+1" will either swing your chance of success by around 10% if your skill is around 10-11 to as little as 1-2% if your skill is around 17 or 18. A +1 therefore means vastly different things depending on your ability, which is just weird.
point buy: Yeah, it's more flexible, but it's also prone to making one-trick ponies.
Overly complex rules: Roll to hit, roll defense, roll damage, subtract armor rating, multiply by 1.5 if the attack is piercing.... etc etc. And that's ONE SECOND of melee. Phoenix Command musta rubbed off on SJ.
Now, having said that, since the rules use inches, feet, pounds, etc, all the sourcebook "crunch" is written in such a way that converting them to other systems is pretty easy, which is good because the sourcebooks are almost universally excellent.