Megaman X is in the game. Sigma is playable for the first time since... Ever. And both are key elements of the storyline after being forgotten by Capcom for so long.
That is ALL I needed to hear: I'm buying this game, no matter what, and I'm mastering the HECK outta my Blue Bombin' Reploid. GOSH am I happy.
Oh and the trailer shows that Strider Hiryu's also back - which I'm happy about as well.