Usually the game server would keep everything in the game instance in memory, for an FPS or RTS. As great as Redis is, it's way more cumbersome than having everything in your data structures ready to go.
Redis is suitable for persistence, so where it would really be appropriate would be storing things like player profiles and other misc data that is not tied to a specific server. In fact, that's exactly what I'm using it for.
I realize that for most FPS or RTS titles, Redis might not be a perfect fit because the dataset is small for each map and the number of players never exceeds some preset limit like 64, but I think Redis might really shine as a means of storing persistent world data - for example MMOs, without having to shard heavily. Most modern MMOs use heavy sharding and can really only support a couple hundred players in the same area or shard at the same time. With Redis, you might be able to handle many thousands of players in the same area without sharding. You don't need sub-millisecond response time - as long as you can do most operations in a few milliseconds, things will be fine, since most players are on ~100 millisecond WAN connections anyway.
You might finally be able to break the model that current large games like WoW use - heavily sharded with all persistent data stored in Oracle. Redis should allow you to shard less and use SQL less, resulting in a much better experience, especially when parts of the virtual world get crowded.
I would be extremely surprised if WoW has issues when lots of people are crammed in a small space due to their Oracle backend. More likely it is all the calculations that the serverside is performing. I know a few people who work on other MMOs and they keep everything in memory, even accounts for logged off users.
I'm curious: what you think (or know?) they are doing that is causing the database to be a limiting factor?
I didn't mean to imply that the bottleneck was due to Oracle. I was saying the exact same thing you are saying - it's due to memory or CPU limitations in their application server, which forces them to heavily shard.
If Redis can help them scale from the small shard to larger shards that support more players in an area, it might be interesting.