Can't fully extend his arms; has a bunch of exposed wiring in his abs; walks and runs as if he has the droid equivalent of arthritis. And you say, well, he was put together by an eight-year-old. Yes, but a trip to the nearest Radio Shack would fix that. Also, I'm still waiting to hear the rationale for making a protocol droid a shrieking coward, aside from George Lucas rummaging through a box of offensive stereotypes (which he'd later return to while building Jar-Jar Binks) and picking out the 'mincing gay man' module.
Yes, I know, I want one too. But I tell you what: I want one with a hand guard. Otherwise every lightsaber battle would consist of sabers clashing and then their owners sliding as quickly as possible down the shaft to lop off their opponent's fingers. You say: Lightsabers can slice through anything but another lightsaber, so what are you going to make a hand guard out of? I say: Dude, if you have the technology to make a lightsaber, you have the technology to make a light hand guard.
John Scalzi's Guide to the Most Epic FAILs in Star Wars Design