I think I'm on 90% completion now, but some of the... What are they called, ability points? Whatever, the scores you need for some of them are crazy. I've just done a special stage in France, gave it a pretty good go, noticed I was getting pretty much no points for drifting for some reason even though I was giving it a good go (handbraking around almost every corner) and I got 300 points for first place and perfect sections at every checkpoint and the full bonus at the end... My score was 3800, the third star was for 5600! I suppose I just need to go properly nuts, completely disregard my position, roll at every opportunity and get as many destruction and drift points as possible.
Also, the combo challenge in New Zealand is really easy, Greece had some much harder challenges than that. The drifting one in NZ is great fun too because it has the little loop bit where you go under a bridge, nailing a decent drift around that is a nice feeling (even though it's pretty straightforward). Yeah, I really like this game, much more than I thought I would.