Want a Survival-oriented zombie map? Check Left 2 Die. Favor a 6v6 space battle theme with no respawns, where each player commands a powerful warship? Star Battle has you covered. Interested in tower defense maps, survival themed Hero games with a Dead Space flair, or a straight-out fantasy RPG? They're all online.
And some of them are good. Arcade has been in beta since the middle of 2012, but I suspect a lot of players returning to Heart of the Swarm for the single-player game will discover it for the first time. It's worth discovering. I've played far more arcade games than other types, simply because there's such a wealth of experiences available.
This neatly addresses one of my concerns with Starcraft II: HotS, which was that the relentless E-Sports focus of the ladder tournaments would ruin the multiplayer game for more casual players. I enjoy playing conventional SC2 against other people, but I'm not very good at it. SC2 itself, meanwhile, is a more complicated game than its predecessor. This drives home with particular force if you were a Zerg player in SC1, when Hydralisks were one of the best all-around units in the entire game. In SC2, their iconic role has been diminished by the advent of Roaches and the decision to limit movement speed when off Zerg Creep.
This last change shifts them from an all-around offensive powerhouse to a support unit with specific uses that requires careful micro to exploit. Many of Blizzard's patch notes and balance posts make repeated reference to E-Sports play as the deciding factor in how units are changed. Is that bad? Not necessarily -- but Starcraft's multiplayer was much easier to dive into. For casual players, the ladder can be intimidating -- and that's why the Arcade shines.