Always on xp rewards would further deter me from playing villain side. I have issues with the base xp rate (xp rate without the booster) being too high even on the hero side, that I feel like I'm skipping content. XP rate should more or less match content, if need be options to boost can be present.
Aside from that several things motivates me to play blue rather than red most times, and they both have to do with content.
In no particular order:
Blue side is much cleaner, red I'm getting the impression of being in a slum all the time, this is a very minor point that nonetheless affects my mindset and one I don't expect anything done about it.
More importantly, playing a villain doesn't feel like playing a villain for most missions. It at best feels like playing an anti-hero or a low key criminal. Sometimes it feels like you're a lackey doing menial labour. Villain missions should be the sort that make me think "I wish a hero was here to stop this, mwuahaha!". Maybe the alignment choice could affect the style of mission you get.
Also the heroes that show up to counter bank missions are ridiculously under powered to take on a team of Super Villains. There should be at least an equal number of heroes to villains and they should work with each other in some way. Maybe give the players some sort of input as to who their "nemesis" is so they tend to be reoccurring during those missions, ideally characters should have a "rogue gallery" so they can grow on us yet be different if we switch characters as to not bore us too much.