Solo mid is probably one of the hardest roles in the game to play well, for a few reasons. You need the last hitting skills of a carry, but you can't rely on a support to help you out against ganks. You also need good map awareness so you don't get ganked. You also have to be good at controlling the lane creeps. (maintaining creep equilibrium, pushing the lane when necessary, anti-pushing etc.) As a carry, you just need to keep the lane in one place, as a mid player, you need to push the lane shortly before a rune spawns so you don't lose XP and gold when going for the rune or anti-push the lane when you mess up your block.
It's often also not easy to know when to go for a gank. On some hereos it's very easy to gank and quite obvious when you should gank (e.g. Nightstalker, Beastmaster, Pudge). On others you're better off farming or only ganking with a rune (Tinker, Morphling, Drow Ranger). Something I think not a lot of people keep in mind is that kills don't give that much money, usually. Unless it's first blood or you get a tower as a byproduct of your kill (or you get more than one kill), you get around 200-300 gold, plus some for your assisting team mates. If you're farming well, you're gonna be at around 300 GPM in the laning phase (or more). That means that unless you get a kill in less than a minute, you will get more farm by just sitting in lane. That excludes, of course, the fact that killing someone will set him back in farm, will help your teammates win the lane and make the enemies more afraid to be aggressive. Still, you should keep in mind that even a successful gank on one person is not that cost effective for you alone. That is especially important for heroes who desperately need gold and can't gank that well, like Tinker (when doing a March build. With 4 levels on Laser/Rocket you can gank pretty well) or Morphling.
