If you have RoS, then yes, it gets a lot better than the campaign.
The Campaign is is just the tutorial to the real game, as it has become. Do not worry too much about the story if you do not like it, since you do not really even have to play it if you don’t want to.
Not sure what you mean by “prettier”. Do you mean graphically/aesthetically? Each Act is very different from the last, so I think there is something for everyone.
Personally, I prefer the graphics in the first part of act 1, but I do not like the Halls of agony, and a lot of act 3. Each part of each act has it’s own mob types. You are seeing lots of Grotesques because you are still playing Act 1. When you progress in the story a bit further, you the mob types will change.
When you start playing Adventure mode, you will get a full mixture of the game acts in each session.