My first proper brush w/ the series ended up 3, and maaaaaaaan did its janky execution of heavy-RPG element in an FPS piss me off back then. I'm ok if the weapons do pittance of damage due to lack of skill points and whatnot, but when they game doesn't even let you cheese somewhat by having solid FPS skills was a huge turn off for me back then. Also, good god, screw VATS.
Then years later I played New Vegas and it was appropriately a brand new game that worked much, much better. Still need to finish 2/3 expansions.
And 4, while lots of gameplay got polished, the base story felt pretty weak to me. Still need to get back to it and finish it.