Much better online service and server capability. Dedicated servers pretty much guaranteed for all major multiplayer games.
Huge Halo fan. Was the reason I bought the OG Xbox, reason I bought an Xbox 360. Hell, I joined NeoGAF solely for the Halo community here.
The controller. I've hated the dualshock since the PSX days. Sole reason I was gamecube/Xbox only last gen. Why I didn't really care for the ps3. I'm glad they improved it, but if have to hold it to be sure, see how it is. I'm still betting the XbOne controller will be better.
Easy to migrate my account. I've been with MS for a while. Still use my Hotmail from 1997 etc. Since PS4 will no longer be free online, why create an all new PSN etc. It'll be easier for me to just continue with my gamertag, my games etc.
Plus, Halo. I just couldn't imagine being a ps4 owner and having a new Halo release and not be able to play it. I love Bungie, I love shooters, but Halo was a breath of fresh air in an ADS/loadout/knife melee dominated shooter market. I love the arena shooter/sandbox gameplay. Map control, stealth, on map weapons, equal starts etc. If Halo 5 sucks, like Halo 4 sucks, I'd probably leave the franchise behind.
Don't get me wrong though, I'd love to buy a ps4. Sony has been great and up front from the start. The ps4 is easily a more powerful machine, and cheaper. I'll probably buy both, but right now, I'd probably get an XbOne at launch. I'm dying to play TitanFall as well.