Splatoon seems to be rather well received so far by many that have been splatting each other online, but it's arguably somewhat light in online content at launch. There's the single player campaign, yes, but in online matches we're currently limited to one default mode in the 'Regular Battle' category - 4 vs 4 with random teams.
It's an oddity of the release that Ranked Battles are still locked, though this was confirmed as the plan well before release. You need to be level 10 to access it, which will likely take a good few hours to achieve, but the mode is still locked for everyone until - apparently - a fixed number of players worldwide have reached the relevant level. As for what the Ranked Battles are? It has different internal rankings between matches and the one game mode (until later in the Summer) will be Splat Zones, in which both teams battle it out over a smaller designated area.
Nintendo of America is being a bit of a tease, however, tweeting a 'Splatometer' that apparently shows the progress of worldwide players getting ranked up.
There are multiple theories over what this delay is all about. On the one hand there's logic to it - as the developers made the choice to only have matches with live players (no bots) it's sensible that a certain user threshold is hit to try and ensure plenty of players are around to fill lobbies at all times. With that said, however, we're getting matches almost immediately around the clock at the moment in Regular Battles, and there seem to be plenty of players, perhaps a majority, that are now beyond level 10.
Another theory is that this is a bit of narrative to cover a careful phased roll-out of modes, to manage servers and avoid too many issues.The online component seems to be solid right now, so this may be timed simply to allow the technical side of things to settle down.
When will Ranked Battles be unlocked? We're hoping within the next 24-48 hours, but in truth we simply don't know.
In our current Poll article for Splatoon, at the time of writing a slight majority feel that the amount of content in Splatoon right now is "pretty decent, though the online component needs to be beefed up".
Hopefully we'll have some extra online variety soon.