Price. If you don't have money to spare you really shouldn't be getting this watch.
Usefulness: If you can't imagine how you would use such a device, you should wait to see how others use it. Don't just buy it because you think its cool because you might find it sitting in a drawer. It's not the kind of thing everyone will need or even have a use for, though some people do and can already envision those use cases. (For me, the "killer app" is being able to read my wife's texts without getting off my bike and ruining my splits.)
Cool factor: The Apple Watch may be cool when it comes out, or it might just make you look like an oaf. Google Glass wasn't cool; do you want people to call you the Apple Watch equivalent of a Gl*******? If your ego can't take the possibility of looking the bad kind of geeky, don't buy the watch.
Glitches: New product launches are never perfect, no matter how much the company prepares. If you're not prepared to deal with the inevitable annoyances and problems, don't get the watch.
Otherwise... I'll see you in line in April.
Edit: LOL, MacRumors won't let me say what people call Glass users. Fair enough!