How about here's a list of champs to avoid, because they generally turn into weak links on a team: (These champs aren't useless, but most people in my experience just seem to screw things up playing them)
Nunu (an overall bad kit for dominion, might help you cap top early, but horrific late game as anything but a chaser)
Volibear (if you're stupid and build him AS instead of as a tank, and I laugh every time someone tries to use his 'Q' to run away)
Leona (if you're stupid and build her pure AP instead of as a tank, also she outputs minimal damage no matter how she's built)
Shen (he doesn't really do anything but ult)
Amumu (can be successful, but can't 1v1 anybody and people try to build him pure AP instead of as a tank, potentially awesome with help)
Galio (he's just not a strong enough mage to win fights consistently)
Janna and Sona (sure they can work, but most people don't know how to actually succeed with these two as damage dealers)
Zilean (You have to know who to ult and when and be ready to run all over the place, most people try to play him like a standard mage)
Kayle (If you're ulting yourself 9 out of 10 fights you're doing it wrong, you deal less damage than just about every AD or AP carry, also Malady, Guinsoo's Rageblade, and Hextech Gunblade are your best friends)
Those champs are difficult to succeed with due to shortcomings in their kits or tendencies for people to be absolutely moronic with their builds due to bad scaling or poor synergy.
Despite what other have said, Eve and Twitch are fairly easy to play. You gank with a teammate where ever possible and target low health enemies for easy kills and back cap when down a point. AP or AD work for Eve too.
Also, as a note (and a personal pet peeve of mine): if you're playing an AP caster Rabbadon's Deathcap should be thought of as a REQUIRED item and not a luxury item
. Build it as your 2nd major item (after boots and Archangel's Staff, Rod of Ages, or Rylai's Scepter). If you don't build it, you're gimping yourself because the enemy team's AP casters WILL build it.
Another note, if you're gonna play Ryze build either Catalyst of the Protector or Tear of the Goddess (recommended) first, finish your boots, then build the other before building other items. You'll get more mana faster that way to ramp up your damage. Only finish off Odyn's Veil and Archangel's Staff (or Manamune, heck both if you want to be cheeky) after getting a Frozen Heart and maybe a Hextech Revolver. You'll be more survivable with roughly the same damage output.