I find the Abbey and Mayor followers and the barn (the barn is not a follower) shakes up the game quite a lot and adds a lot of value to the gameplay. The last time an expansion did this as way back with Traders and Builders.
Forgot to include at the start of this - that we are currently playing with the Basic game, River 1 & II, I&C, T&B, K&S, GQ11 and A&M as a regular playing set and have set the other expansions aside for now. With only those sets, then we are setting a regular meeple aside and playing with a set of 12. We are also playing games with five players.
I agree with Joff and Novelty that the Abbey and Mayor pieces have been a great addition to the game.