City of Steam is a browser-based game. But it doesn't match traditional notions of a browser-based game. It's a fully 3D game, with pretty decent graphics, and is very responsive. Mechanist Games is focusing on making as easy to jump in and play as possible, with very fast loading times. It's actually quite an impressive technical feat, in my opinion.
The setting is a more steampunk-ish, Victorian or Industrial Age fantasy. There are humans, elves and orcs, but the orcs and goblins form the working underclass. It's similar to Troika's Arcanum: Of Steamworks and Magick Obscura.
Mechanics-wise, it looks to be a themepark MMO using the trinity. The main classes are Gunner (ranged dps), Warder (melee dps/tank), Arcanist (mage), and Channeler (healer). Gameplay is quest driven to start, but there are lots of dungeons. Also interesting is that the dungeons have challenge modes, which change the objective of the dungeon. One challenge mode might be to kill 200 enemies in 5 minutes, or break 100 barrels, or make it to the end before the timer.
I tried out a Gunner for a bit. The animations were nice, and the early part was fun. I didn't get very far, and only got a peek at the customization options, which look very extensive and looks like complicated fun. People who enjoy tweaking their characters should enjoy this aspect.
Somehow, even though it is a 3D over-the-shoulder perspective, I got a very "Diablo II" vibe. I'm not sure entirely why that is so, it may just be how the UI is laid out, and the fact that there's lots of barrels and crates to break. But that's probably a good sign.
In keeping with the extremely accessible theme, City of Steam is going to be a F2P game.
All in all, City of Steam looks like it will be an interesting game. It's definitely worth checking out, especially to see how far the brower can be pushed, and how quickly you can just jump in and start playing. It really drove home just how much more powerful modern computers are.