OK so I'm sure the achievement for freeing 1,000 snow fairies came with a prize, but the achievements system is messing up a bit and won't tell me what that prize is, despite showing the star symbol on the thing. x_x If anyone can tell me what the prize is, I'd be much obliged.

You get the Luminaire site theme. (Source: this forum topic)

If you don't know, you can see your themes and change it here: here, also found by going to your User Dashboard and clicking on "Themes".

I also seem to recall the star showing the prize being broken for a while? I can't see it either when I click on it.

Thanks for the answer ^^

The star thing has been broken for a while, but I forgot to look into whether it was just for me or not. x_x