I'll probably play the single-player mode first just to get a good grip on the controls, but I don't think there's much doubt that this is, in fact, a multiplayer title through and through.
Turf Wars so far has been deliciously fast-paced and hyper-aggressive, which is great for some quick fun online. Splat Zones probably has more of a defensive mindset, which is fine too, but doesn't excite me quite as much.
I always enjoyed playing Payload on Team Fortress 2, so I'm very much excited for the Tower-thingee.
Of course, even more excited for when they release the option to form teams and take them online. It will be a bit of a bummer to do without until August, but I guess we'll just have to make do until then.
...I'm just pretty excited in general.