[Update: Seems there was a bit of confusion and PlanetSide 2 will NOT be cross-compatible between PC and PS4. DC Universe Online is a still a go for cross play, however.]
As amazing as the PS4 conference was, Sony was able to sneak in a tiny little detail: online play is now locked behind PlayStation Plus. A bit of a bummer for people who saw free online as a big selling point for PS3 over 360, but Plus is rich enough that you may not mind it all that much.
However, there is an asterisk next that new requirement stating that some games will be playable online without a Plus account. Two such games are the free-to-play MMOs DC Universe Online and PlanetSide 2, scheduled to release on the PS4 some time during the launch window. Both games DC will also be cross-compatible with its PC counterpart, although the PS3 version of DC will sadly continue to be locked to its own platform because of hardware differences.
Superhero brawling on one side, continent-spanning FPS action on the other, all truly free-to-play on Sony's new wonder box. Ball's in your court, Microsoft.