Early Gen 1's (or pre-Gen 1's) the trigger return spring broke (what a PITA)
a trigger spring is only $2.50 so regular maintenance of the pistol should be done at say 5k rounds...replace the recoil rod/spring assembly ($7.50) and replace the trigger return spring ($2.50)..... every 20k rounds replace the striker spring.
of course "if" the trigger spring breaksduring usage most people will not even notice it because the recoil of the gun will reset the trigger, most brokwn trigger springs are found during dry fire when the trigger will not reset, all you have to do is push it forward though and it will reset.
as far as what generations to avoid, I would say the Gen 4 right now... in the 9mm pistols there has been some talkthat the dual recoil spring setup while optimal for the .40 caliber is proving to be a problem in the 9mm shooting "lighter" factory loads.... using nato spec 9mm seems to fix the problem or +P stuff, glock supposedly has a new lighter spring setup available if you phone them and ask for it, or glockmeister has a replacment guiderod assembly that allows usage of the original flat spun springs from ISMI or factory gen 3 recoil springs.
the gen 2's are great guns for people who do not like the finger grooves, but then your missing the light rail (brownells just came out with a rail system to retrofit gen2's), the gen 3's are great.... finger grooves and light rails.... the gen 3.5's are even better with the standardized locking block between the 9mm and .40's.
frankly the best of the bunch in my opinion is the gen 3.5's with RTF or RTF2.... very grippy and easyto hold, seriously with the new gen4's and the rtf gen 3.5's I think glock is going to put the houge/pacymer grip socks out of buisness, makes the gun very easy to control with sweaty hands.