I can't think of anything they removed that people didn't notice. In fact, tons of people noticed, and complained.
But GTA IV was a new start. It wasn't so much removing features, as not putting them in. It was getting to grips with new hardware, performance capture, physics engines, graphics engines, production etc. It was a total rethinking of core elements, because San Andreas 2 simply wouldn't have cut it in 2008 on PS3 and 360.
It was a good game. It wasn't feature packed, sure, but it did come out in 2008. It was a technical marvel when it released, and a good open world game. It was a necessary stepping stone in creating GTA V, like Red Dead Redemption, Max Payne 3 and all the other Rockstar titles than have released since 2008.