I would never go to Koh Tao as a dive destination, this being said after I've spent nearly a year there.
For dive training, no place like it. You get a lot of bang for your buck!
Escape from the hustle and bustle, Koh Tao has a really cool lifestyle!
Dive quality, I'd give a 4 out of 10 (10 being highest), but then, I've seen four whale sharks here. That is kind of hard to beat.
You were asking about small groups, I'm laughing out loud, there is no such thing!
For a good patient DM, avoid the larger shops, they tend to be full of kids working too long hours and drinking hard at night to make up for lost time.
Free hotel transport, all the large shops provide it, (for that matter I'm sure every shop on the island does), you really don't need it though as the place is so small, you'll be wanting the walk just to stretch your legs.
Enjoy