I have 2 OTTB's and one thing I've noticed is that they have a short attention span. You can't just trot around the ring 6 times asking for something. You've got to be confident in what you want, and if you don't get what you ask for in a reasonable amout of time, go back to the walk, or do something that you can praise him for. With no praise, they will become bored and then everyone will get frustrated. Also, I agree that you need a strong base for the groundwork. Lots of OTTB's are super smart, so if you start right, you will have an easier time when you get on. I love my OTTB's and I will never own a different breed :) They can be super fun, or super frustrating- just remember: Eventually you will hit a wall and it will seem like you take one step forwards and 3 steps back. It's just about finding what works for that particular horse.
The nice thing about OTTB's is that you will never have a "go" problem and most of them are ok with water (on their faces too), trailers, and saddling at a young age. They were started at the track which could be good, or it could be bad (always the chance of picking up bad habits). In my case (with my 4yo OTTB), it was good :) She is the most responsive horse I've ever ridden and I love her.
Good Luck with your search, and I think it's great that you're thinking of rescuing :)