Main character OCs do run the risk of being < gasp! >
Mary Sues.
It's why I enjoyed Gerry's "Date With Superman" so much. Rachel was very much an OC, but she didn't discover Superman's ID before (or even after) Lois, come from Krypton, wow everyone with her brilliance, investigate a mystery with more efficiency than Lois, discover a familial relationship with any of the main characters, reflect back on her close personal relationship with any of the main characters in the past, rescue Superman from Kryptonite, or even fall in love with Jimmy.
(Of course, Tank prolly wouldn't classify that last as Mary Sue behavior...
)
I'd say villains, particularly original villains, are the hardest characters to do right. I only read the first few sections of Yvonne's Purity, for example, so I can't say whether or not the characterization lasted all the way through,
but her villains there were superb. Not insane mwa ha ha! types, but rather people with their own agenda, prepared to coldly and ruthlessly pursue it. Most bad guys don't think of themselves as such. Well, except for Tempus. And Lex, who was darn proud of it. :p
