Last week at Comic-Con, SOE showed off the latest build of DCU and spotlighted a new feature with the awkward name "Play as Iconic." ("Iconic" is what the licensed DC comics characters are referred to as in the game.) In the main game, you will play as your own custom characters, getting guidance and help from established veterans like Batman and Superman. But, as you've probably deduced, in the Play as Iconic bonus mode, you'll get to play as DC heroes (or villains) in a 2-on-2 PvP match.
The mode definitely feels tacked on -- an SOE representative told me that "it's just really to fill out all of the options and playstyles of what people would want to do." At launch, Play as Iconic will only feature characters from the Batman franchise, which is probably DC's most mainstream draw anyway. As expected the Iconic characters will have unique moves and powers, distinct from your player-created characters, but you can earn PvP currency while playing the mode, which you can spend on gear for your custom supers.
I played as The Joker in the Batcave, the mode's only map (at launch). The arena is furnished with memorabilia from the classic depiction of Bats' HQ, including the T. rex and giant penny, along with a nice display of historic Batman and Robin suits. Joker can toggle between a melee-attack "crowbar mode" (as made infamous in the "A Death in the Family" story arc) and a ranged-attack mode in which he trades his iron rod for dual irons -- useful for holding down the map's control points with a teammate.
Not surprisingly, I found that combat in the Play as Iconic mode was plagued by the same issues I've noted in the game at large: hits feel floaty and movement isn't very fluid. Additionally, and further cutting off this mode from any narrative thread from the main game, players can choose the same characters, which means you could pit Batman and Batman against ... Batman and, yep, another Batman.
If you're already sold on DC Universe Online, Play as Iconic is a suitable bonus -- a diversion from the main game and a chance to control some of Gotham City's finest and most fiendish. But it's far from a game-selling feature and, especially if you don't nab Batman through a pre-order, the mode really comes off as an afterthought.