I'll echo the two most common ones already noted: music and language.

Where music is concerned, I've owned mandolins for a few years now and have made little progress; I'll play for a few weeks, and then they'll gather dust for a month or two. This year I started off quite well - I learned a bunch of new tunes. However, I then decided it would be best to switch from directional to alternate picking, which essentially meant starting over and re-learning everything from scratch. I found this so frustrating that I have yet to properly go back to the mandolin. I've also had a friend's bouzouki for the past few months and haven't learned anything on it.

Language is slightly worse; at least with the mandolin I've made an effort. At various points I've wanted to learn different languages - Norwegian, Spanish, French, Polish, Italian, Russian. I did French back in school and studied Spanish in my first year of university, and of course learned Irish from way back in primary school. All three I've at various points really wanted to learn properly, but never actually gotten around to doing anything about it so far.

There are lots of other things I think about learning - how to program, how to knit, how to ride a motorcycle - but I have yet to consider them as properly as the above. ("Properly" used loosely here.)