Okay, this idea has come up for the 3rd time in conversation now, and it's been bugging me for years. The idea is a game where all players make decisions/issue orders to units while the game is in a paused state, and all the actions are executed at once, in real-time. This could be at any level from a TRPG (i.e. Final Fantasy Tactics), to a small-scale skirmish (i.e. Worms), to a game with bases and many units (i.e. Advance Wars), to a Risk-type game (i.e. Risk).
The main disadvantage to this I can see is that the player doesn't see the result of an action immediately. In a traditional turn-based or real-time strategy game, the user clicks "attack that fool" and there's an explosion straight away. There also might be weird situations (you send your unit to melee a unit running in the opposite direction, for example), but that's a low-level detail; a good game designer should be able to get around these things. Finally, there's the general paucity of turn-based games, but they offer a level of tactical depth impossible in real-time strategy games, and there's always a market for them (dozens of new TBSes appear each year).
- Are there any games out there that use this paradigm?
- Why aren't there more?
- What would be necessary to make this work?
- What potential issues could you foresee?
To clarify, I mean pure strategy. The commands are input beforehand, and the resolution is realtime, but it doesn't require realtime "twitch" input by players.
EDIT: Iain's answer pointed me to Laser Squad Nemesis, which basically answered the 2nd and 4th question ;-P -- If you want to know why this won't work, download the demo (It's 9MB) and click the first tutorial. OUCH! Of course, Laser Squad Nemesis's UI doesn't help (play and pause buttons? seriously? and why do I have to click "issue orders" every turn if that's the only thing I'm going to be doing?)
For simpler things (like the board games mentioned in a couple answers, or Zwok that Iain created), this paradigm works great! Issues seem to arise with a more traditional tactics/strategy game, because the lag between the entry of a command and the execution of the command is apparent. In fact Zwok - and some of the board games mentioned - work because off this lag, not in spite of it. You're encouraged to guess what your opponent will do next and "think ahead". However in a more traditional small-scale tactical strategy game, the frustration factor isn't worth it.
It might be interesting to see a game like this on a really large scale. For example, Lee and Napoleon didn't order their troops around in real-time, but rather made decisions based on information that was often hours old -- and it often took hours for those orders to get executed. Even today, generals don't have real-time top-down maps. But taking that "frustration factor" out would require some seriously deft game design.