The mechanics of a lot of modern FPS games (especially their multiplayer modes) often employ mechanics, which evolve around three common aspects, which are linked together:
* move efficiently through the world without being an easy target
* discover enemies, or generally decoding the visuals
* weapon handling and knowledge (like timings)
The "decode visual" aspect might be least obvious for shooters, so here's an example of visually very dense Battlefield 3 map:
youtu.be/628bRlLS5M0
Those three aspects - not always realized with the same mechanics - work well together and have been used in a lot of games. Not just in 3D but they worked just as well in 2D. Vertical shooters like Tyrian or Raptor just put less emphasis on movement and weapon handling:
* movement is in 2D but still tricky due to enemies & map features
* decoding the visual effects in real-time requires training
* the weapon handling is relatively simple
I think this is obvious, but here's a link to Tyrian gameplay:
youtu.be/tQdu4NsWU4Y
Or if we take a look at Quake. They are actually doing something smart, which the modern shooters ofen don't because it's harder to map to a realistic theme: Quake cranked up the movement aspect. To the point that movement became much more important than the other two aspects. Here's a quake movement tutorial:
youtu.be/dS7u2lK7dk8
An interesting mix are the "Worms" (round based) and "Soldat" (real-time multiplayer) kind of games. They are 2D but look like a 3D shooter from the side. Compared to the vertical shooters like Tyran, the visual aspect is less strong but on the other hand the movement is quiet a cognitive load for the player because there are multiple levels and not just the occasional obstacle as in vertical shooters. The more complex map features also make weapon handling more difficult.
They player is constantly busy targeting and moving in this soldat gameplay:
youtu.be/HhulGe6L9AQ
Can anyone think of more examples? There must be whole genres, which I didn't think of. I'm trying to make something feel like a modern shooter but with simple visuals. So far, I have captured the weapon handling in a small demo: http://apps.gamejs.org/cratewars/