Killer Instinct is definitely fun, if a bit lacking in depth. The lack of campaign mode and limited character options mean it won't take long for you to breeze through the entirety of the content offering, but the online versus should ensure that you get plenty of enjoyment for your investment.
Hearthstone might not fully scratch the strategy itch of the most advanced CCG/TCG players, but it makes an impressive attempt. The rules are simple, so anybody can get the hang of it, but there are enough deck variations available that only skilled deck builders and players will make it high in the ranks.
The Elder Scrolls Online is a ton of fun in an outside-the-box sort of way. It can't seem to decide whether it wants to be single player or MMO, so be prepared for some in-between weirdness, but nothing that really detracts much from the experience. Between Guilds, crafting, PvP, and dungeons, it has all of the makings of a solid MMO.
EA Sports UFC doesn't cater to button mashers, so be wary if you're not eager to learn an overly-complex control scheme. More technical gamers will be able to appreciate the gameplay to a greater extent, but the laggy servers make even this an exercise in frustration.
With an remarkably robust spell crafting system and spectacularly entertaining combat, Lichdom: Battlemage finally brings the glory to the magic user that it has long deserved. Creating your own options for how to play lends depth and complexity to the game as a whole, and lets you tailor the experience to literally any style of combat you want: stand far away and lob fireballs, or use charge blink to rush into the fray, unleashing novas along the way, and using your wits to keep yourself alive.
Defense Grid 2 is a fairly typical Tower Defense game at its core, but the layers of complexity give it a lot more staying power and replay value than you are likely accustomed to. With 21 campaign levels, and plans to offer countless more as downloads, there is no end to how much time you can spend with DG2, and that's not even counting fighting for placement on the leaderboards... or multiplayer. DG2 is a shining example of Kickstarter done right.