There are two reasons to send a hero mid for the levels or for the farm, so gankers and carries. A lot of melee carries can't really compete with usual mid heroes though, so you're gonna want to send those with supports unless the player is extraordinarily good. --- If you believe in Cthulhu and are 100% proud, put this in your sig. http://img.photobucket.com/albums/v723/AJSadler/Sigs/CrimsonJesus.jpg
A solo level-dependent carry (storm spirit, invoker) A solo level-dependent ganker (queen of pain, pudge, beastmaster) A hard carry with a support (antimage+lich)
The middle lane is hard to gank, easy to push and gives easy access to the rest of the map. If you're going mid you should have something in mind that plays to those features. --- And with that...pow! I'm gone!
Very often, it is a strong ganker or semi-carry that can lane well without support and loves having a level advantage... Pudge, QoP, PotM, Nightstalker, Clockwerk, Tiny, Shadow Fiend, and many more. Rarely, you will also see a support + carry go mid, but this doesn't happen often. --- [EVe *also responds to Santana*
heroes with strong rune control are popular picks too
miranas stun+leap, shadow demon and obsidian destroyer's banish abilities, etc etc, all let them beat their mid competition to contested runes --- I hated going to weddings. All the grandmas would poke me saying "You're next." They stopped that when I started doing it to them at funerals.
For me, anyone who is level-dependent should go to the middle lane. Bane for example, should get to 6 as quickly as possible. Not only can he kill his middle opponent quickly, but he can easily gank either top or bottom with ease by just using his ultimate.