I think the Droid X's "killer features" are plentiful. Some are noticeable, some not so much.
-Screen: 4.3", which means everything is easier to see, and typing will be a breeze.
-Processor: 1ghz, which of course not that unique nowadays, but it'll be speedy. What makes the DX unique is that it has an OMAP processor, which means we'll see better 3D and battery life. Speaking of...
-Battery life: Hearsay at the moment, but everything I've heard is that the battery life on this thing is a beast.
-Camera: Every shot I've seen from this thing has been fantastic. I think it's on par with consumer iPhone 4 shots I've seen (meaning not the gussied up, ideal conditions, taken by a professional photographer official shots from Apple), and it has a higher resolution than said iPhone camera.
-Assorted bells and whistles: Things like the numerous bundled apps, included 16GB micro SD card, and the 3 (THREE!!!!) mics aren't just a cherry on top, it's an entire orchard of 'em.
And finally
-Android! Eclair imo was already the best mobile OS on the market. The imminent Froyo update, which will make the best mobile OS on the market into an untouchable one, will make the Droid X the creme de la creme of cell phones. Even if one of those 98% of iPhone users that use their smartphone to check Facebook and email once a week never know.