Let's see, I went from "zomg I can tame everything" to "dang, I only have like, 4 spots left and cata hasn't even COME OUT YET"
The ones I set free:

They were both pets that Aldera had at some point during her leveling to 60 back in Vanilla, and I retamed them on day 1 of 4.0.1. But I just wasn't going to pull them out, so I let them free.

I picked up Shy and Sian at the same time, and I'm more likely to keep Sian. I grabbed Shy because I love saber models, and at the time there weren't a lot, even on beta. But now there are just too many, and I prefer Rak'shiri and the more realistic looking cats.
What's on the fence, and might have to go come Cataclysm:

Aotona I tamed because I've always loved that color. I'm very much a purple and blue fan, and the minipet version has always been my favorite (plus it was a gift from my boyfriend). But she's sat in the stables more than she's been out.

This one is my favorite color owl, despite the harassment that I used to get from the Monstrous Kaliri in Skettis, so I grabbed one in Sethekk during one of my Raven runs. Still, I don't use birds of prey much.

Nura is my favorite looking Chimaera, and I spent a good week looking for him. But, it's a bit offputting how large he is, and he tends to get in the way because he flies so low. (Oddly, I didn't have as much problem with my Silithid Colossus on beta when it was that big... I seemed to maneuver around it just fine). I'll probably hang on to him though, just to have a Chimaera.

I picked this one up on my very last ZG run, mostly for nostalgia's sake. But I really don't need 2 wind serpents and I much prefer Seiryu, my ZG Soulflayer that was with me all through BC.

I prefer Bait (Grub'thor), but I wanted to try a jormungar now that there was a skin that I liked. But I think Bait wins.