So? :P
What qualifies as a milestone in gaming for you?
More content? Upgraded graphics? More sales?
It's all about the actual influence it has, and the way it actually changes the perception on certain things. New things, instead of upgrades. Different things instead of more of the same.
I'm not saying SA isn't good, I'm not even saying it's worse then the others. It just isn't a milestone in the gaming scene, rather an improved and expanded version of the old one.
Oh, and I still enjoy GTA III and VC (VC's my favorite - SA constantly turns me away as it just doesn't manage to draw me in. VC does). I can see why one would qualify III as boring, though. The other games have so much more to do, to work for and a real story to boot. Still, I like to create my own things to do, set my own goals and write my own story, so I enjoy III just fine as well
Even more so because Liberty's more my kind of city.
Oh, and (as a different issue)
Remember, GTA:SA is one of the buggiest GTA's of all time :P
Which I personally think to be very, very odd.