AP master yi, the Assasin, Soak-offtank, and Anti-carry
Many people think AP yi is a troll build, and not really viable but i think its decent, and he offers a bit more utility and more team fight wide burst without having to be directly ontop of the squishies. He can tank towers at full hp better then most all champs which is excellent for helping your team mates dive or pushing. AP master yi also has one of the easiest and best laning phases with a 2000+ range nuke that can harass from a completely safe range, in addition to extreme healing with meditate.
Most AD yi's use ghost ignite, or ghost cleanse which is pretty solid for yi, but with AP yi you really don't need ghost for auto attacking while running, Flash ignite is much better for increasing the range on your 2000 range nuke even further and escaping ganks and walls and such, as well as further improving the laning phase and kill possibilities with his nuke + ignite.
This should be one of your favorite parts of AP yi, AP yi is very easy to lane with and can shut down solo's (with some skill) or duo lanes with easy.
Begin laning by using alpha strike as much as possible to safely harass by bouncing off of low caster minions to last hit them while harrasing, you should have no mana problems and if you do get harrased down below half don't be afraid to meditate, It takes some practice but after a while you will "feel" how much damage you will be able to do and know when you can flash in alpha + ignite for a kill, Most people will not expect it, If you're not able to get any kills don't worry and keep up that easy farm with alpha strike, although be careful to not push your lane by spamming it too much.
Champions that will poop on your parade are generally anyone who saves ignite for your meditate, and champions like gangplank, katarina, MF, and Tristana who can reduce your healing by 50% with out ignite.
Champions with high CC like Cho'Gath, Blitzcrank, Janna and Alistar can really ruin your day as well.