I've ridden 3 thoroughbreds in lessons. Two of which were ottbs.
First one was an ottb, around 24-26 years old if I remember correctly. He had enevr gotten to the track as he was too slow. He was probably the calmest horse I've ever been on, it could be thunder storming and he would act as though it was a bright sunny day. He wasn't a very good mover, and had a few leg issues, but otherwise, a very sweet horse. All the beginners loved him. Unless someone told you he was an ottb, you would have never guessed. A 28 year old arab was wilder than him.
The second wasnt an ottb, I think he did eventing at some point. He was very fat, and was a terrible mover (not leg issues, just not very nice paces) but he was very sweet. He acted up more than the ottb, but he was generally very safe. I loved riding him, though he was tough, he was great to learn to trot on. He gave me my first jump :)
The third I rode recently, I actually ride him now. He's a 10-20 year old ottb, no idea of his age but I'm estimating. He's great to ride, very responsive, great mover, wonderful horse. Out of the three thoroughbreds I've ridden he acts the most racehorsey. While tacking up and ridding he's fine, but when I walk him through the stables, he will flip out. He wont always, but often enough that you have to be weary. He will spook a bit, but not badly, he will just stop and maybe jump around a little.
Overall I like riding thorughbreds. Have never had one that acts really racehorsey, but I. Sure ill encounter one at one stage.
If I am not riding I'm thinking about horses, and if I am not thinking about horses I'm riding