So I was dinking around, and checking out some new games, and I noticed that, the idea of the MMOFPS, or connected FPS or whatever genre that most closely matches destiny is exploding. By my count, there's one released, and 4 upcoming games that look to hit a lot of the same points as destiny. Most of which are already in beta, and can be played now.

Caveat emptor, we know but grains about destiny past it's nature as an always connected FPS, and it's interesting setting.

Planetside 2

Sony's massive PVP FPS. It features crazy huge battles, 3 factions, and constant war. It's F2P, with your standard "Speed up the stuff acquisition" model. It's out now, and you can go ahead and jump in and get started. Planetside 2 is commonly compared to Destiny because many expected bungie to go more in a massive PVP route than destinies connected co-op route.

The game seems to have a fairly devoted fanbase, and there are lots of updates coming down the pipe. The biggest problem is the fairly bland metagame, which serves to wipeout the sense of faction progress every time you play. Supposedly it's pretty great with a dedicated player group, but I haven't been able to find one.

It's also one of the least accessible games I've ever played. The tutorial is a link to 2 hours worth of youtube videos... thanks but no thanks.

www.planetside2

Defiance

MMOTPS (third person shooter), connected to the SYFY show of the same name. Releases next month.

Really not sure on this one, it's Buy to Play (No subscription) like Destiny. It's third person, and it features PVP and PVE. Personalized vehicles also seem to play a big role. It seems to go directly for more MMO tropes, but it relies on a skill based systemIt's big hook is that somehoow what happens in the game will influence the show. Crazy idea is crazy! There are also regular massive events forcing players to compete for suddenly appearing resources.

PC beta keys for the beta here, http://www.mmorpg.com/giveaways.cfm/offer/424/Defiance-Beta-Key-Giveaway.html

Firefall

Third person, F2P currently in beta

Firefall is interesting, as it follows a similar free to play mode, has MMO quests, but the multiplayer is locked around specific builds, not player builds. There's a big focus on esports, and the developers are agressively updating it.
http://www.firefallthegame.com/

Warframe

Another with an open beta, F2P shooter/slasher. Looks interesting, and it's pure PVE. At the moment, there doesn't seem like there are a ton of content outside "Do mission, upgrade dude, craft stuff, treadmill level, keep going."
https://warframe.com/

So, it seems like the Massive shooter genre is opening up. Given what we know about destiny, it seems like it has a few ways it can stand out.

1. Get away from standard MMO or Cod style "fill bars!" tropes.
2. Lots of variety in quests. Kill x dudes, or get to x then get out seem popular, but I think everyone is tired of those.
3. An interesting meta game beyond just capturing territory, or leveling a character
4. Really high quality combat, from everything I've seen the actual quality of the combat can vary pretty wildly. I don't expect the central combat loop of destiny to be as tight as Halo, but it seems like there's ways to improve it. Though, Warframes Melee combat looks quite good.
5. PVP as something more interesting. I think Defiance's race for resources is really cool, so hopefully destiny has some interesting mechanics.