Grow a beard - while I have very coarse stubble (many men whinge about pash rash after a session with me), I can't get it to form any sort of reasonable looking facial hair. I'd love a goatee, but it just looks like a rodent has been trying to form a nest on my face.
Sing - tone deaf I am. I used to have to do a singing component to my music exams as a teen, and I had one examiner remark "I've heard dying elephants sound better than that". Karaoke is not my friend, but how I wish it was.
Cook a roast - I am a good cook, I really am. So cooking a roast should be simple. Turn on the oven, bung in the slab of meat, chuck in the veggies, make gravy, enjoy. But no.... the last time I tried a roast it took forever and was fucking horrible, so I went and got subway for everyone.
Run - well I try to run from a fitness perspective, but it's more of a shuffling wheezing, death jog where I'm all hunched over like an arthritic old man. I do hope with time I will be one of those guys you see running down street looking like they're going for a simple stroll. But I just don't think it will ever happen.
Pronounce words the containing the "th" sound - yes, I actually have a speech impediment. It's more of a lazy tongue thing though. Where as normal people's tongue will automatically go forward to touch their teeth when pronouncing any word with the "th" sound, mine just flaps about and does weird shit. While I can do it, it's a conscious effort to do so. "Brother" is the word I have the most problems with. It usually comes out as "Bwuvva".
Accept compliments - I still blush and defer to other people/circumstances as being responsible. "Oh no, it was all Jesus/flesh eating virus/hot sex with your mother that helped me lose the weight. I had no input into it at all."
I'm sure there a million other things that belong on the list (I'm sure the ex-boyfriends would add a few more too).
So what simple things can't you do?