The introduction of weather effects has added a significant boost to the atmosphere of the game, as you get similar weather effects to what you had in their MotoGP and SBK games; rain, cloudy, clear etc. Visually the looks really good as well, there's a greater sense of depth in the colour of the game.
In terms of content you get even more stuff than in previous MXGP games; tracks, bikes, customization options, and championships. There's more of everything. So if you are a single-player gamer like myself there's great value in this package.
The gameplay physics is not as tricky to learn as in the previous two MXGP games, it feels easier somehow. However this is offset by having, in my opinion, stronger and better AI.
Technically the game has worked really well for me, although others on Steam have had a few issues, which you should probably read up on before buying the game. Personally though, I've not had any noticeable issues after 10 hours worth of gameplay.