Generic rules sets. Rather, it's not the rules sets themselves I hate, but the belief that you can do anything with them and have it work out well. Really, if you think you can do everything well with GURPS (or Hero, or Savage Worlds, or whatever), you're not thinking hard enough.
How did this pet peeve come about? It was actually seduction rolls. Let's follow the logic. You seduce someone by adding together skill, talent and luck to overpower versus the target's resistance, usually some attribute and maybe a roll. If the target is resisting, isn't this sexual harrassment, not seduction? If seduction isn't a major issue in your story, then it's easy to overlook these quirks, but if it comes up a lot, I don't want to fight or dodge the rules every time.
Even the differences between combat and overcoming obstacles often inspires different mechanics (opposed tests vs. target numbers). This sort of thing is why the mathematical field of game theory has more than zero-sum games.
You want to make/play a generic action ruleset? Cool. Call it a generic action ruleset. Want to make/play a generic romance ruleset? Ditto. Sell me on the strengths of the rules. Don't try to sell me by claiming that it has no weaknesses. This applies, not only to game designers, but to people wanting me to join their games.