Well, I'm a huge fan of the original 80's Voltron anime, and I thought a *lot* was wrong with V3D.
1.) Weak Characterization: All the characters were pretty one-dimensional. Keith was the straight-laced hero; Lance the reckless daredevil; Hunk, the lunkhead with a heart of gold, etc. The 80's Voltron Force may not have been the most developed characters either, but they had a depth that V3D lacked.
2.) Supporting Cast? What Supporting Cast?: Many major supporting characters from the original Voltron seemed to just fall in a black hole without any explanation. I was glad that dead weight like Nanny and the Space Mice was gone, but Sven (the original pilot of Blue Lion!) and Princess Romelle were cast out as well, and they were two of the series' strongest characters.
3.) The "Reformed" King Zarkon: Bull$%#&. The guy was responsible for genocide, slavery, and other atrocities, and they made him a flippin' Secretary of Defense! And then he shows his true colors, and it's like "Wow! I never saw *that* coming!"
4.) Amalgomous: While the idea of The Galaxy Alliance being run by one measly little robot was good, Amalgomous was too bloody annoying for words. I rejoiced when they took him apart.
5.) The So-Called "Humor" of V3D: Awful, just plain awful. When ever they tried to play something for laughs, like the recurring newscaster and the two bumbling criminals that worked for Lotor (danged if I can remember their names) it always fell flat. And some of the plots were too ludicrous to take seriously.
6.) Stealth Voltron: 'Nuff said.
Sorry for the little rant, but that's my piece. I wanted to like V3D, I really did, but it had too many bad points. However, in the show's favor, I thought it was brilliant to cast Tim Curry as Lotor.
"As I bit into the nectarine, it had a crisp juiciness that was very pleasurable, until I realized that it wasn't a nectarine at all, but a HUMAN HEAD."