6 added 12 tracks. It reduced the stickiness of grass and stopped it slowing you instantly to zero in most cases. On the Top Gear track it still does, but that's mainly because they didn't really bother to update it.
6 also made tyre walls behave less like actual walls. Tyres can be dislodged in the loose piles, and the tyre walls bounce you off rather than letting you coast around them - Long Beach becomes even more of a pain.
6 added night racing for several tracks, and rain racing for several tracks. The choices seemed random but they had excuses. Puddles on the track never changed over multiple laps.
6 added mod cards that you can apply in single player races to do various things - give extra credits/xp, give extra grip, etc.; and then there were some that added challenges for increased bonuses. You could buy random bunches of mod cards for credits, which was maddening for the fact that several of them were needed for achievements.
6's campaign did give the impression of more advancement than 5's, but it was still a grind for the most part. 5 just had nowhere near enough tracks - 12 at launch and like 600 championships with 10 core tracks + 7 bonus tracks each - totally ridiculous. In 6 they did theme things for car types and tracks sometimes, but there still aren't enough tracks or sport types represented to do that in a big way. It also required you to podium in every race in order to advance, which was poorly received by many, especially as at launch the drivatar AIs had a bit of a runaway lead thing going on.