The main problem I had with this topic from Long and Short Reviews was narrowing it down. I can think of plenty of things I wish I could do and can't, many of which I've attempted and failed miserably at. I did, however, manage to get it down to three.
Songwriting. As a moody teenager I spent a lot of time trying to write songs. I wasn't as bad with the lyrics as I was with the music - I can't play any musical instruments, so was stuck trying to pick out notes on a keyboard - but, looking back, they were angsty, badly rhymed nonsense. As was demonstrated when I showed one to an ex and his response was "Well... you've certainly put a lot of effort into it, anyway."
Dancing. I can just about stick to a beat, but that's it. As a child I wanted to be in stage musicals, but while I could act and sing quite well, I can't remember a dance routine for the life of me. I also have no balance or elasticity - I can't even do a high kick.
Rock singing. I haven't sung in years, but I used to be a classically trained singer. The snag with that is that, while I could manage classical songs and some musical theatre, I couldn't do pop (apart from LeAnn Rimes) and really couldn't do rock, which I love. My singing teacher thought I'd be even unhappier if it was the other way round, and maybe she was right.
So what skill do you wish you had?