Nothing. And I'm not a skinhead/bald git either. I learned the scissors and clippers myself in a 3 way bathroom mirror in service of a fairly advanced stylized hairstyle, if I do say so myself. I got sick of barbers doing my hair wrong; going through constant cycles of it being too short/wrong for my liking, a rare happy middlezone, and then too long/unworkable for my liking. I now consider self-haircutting not much more tricky than styling my hair. I've been doing this since about 2011 and the money I've saved must be enormous. If you're hesitant about going to town with scissors on the on-top mop, there's something to be said for just getting clippers and doing the back/sides yourself. That's what noticeably starts to grow out the fastest and needs correcting the soonest.