quote Chaosfire
The only exclusive pieces of the game are custom textures for you character called "skins" that change your characters appearance and animations. The community is rather fond of these, but they have NO effect on gameplay.
Surprise Party Fiddlesticks made me laugh heartily the first time I saw it. That's an effect on gameplay.

Same thing with reverse Annie.
As for buying things with Riot Points, I only ever buy things that I like (skins and champions) if they're on sale. I picked up Volibear, Brand, and Reverse Annie for 487x3 and saved 12600 IP. Although that's only if the champion has appealed to me before (champs like Kog'Maw or Fizz have not, so I skip them). I've dropped $35 on this game so far but still have most of the points left just because of shrewd buying.
As for gameplay, you can argue that you can buy a powerful champion at 6300 and overwhelm scrubs at lower levels, but besides that, not really.