These days, Raph and a number of other developers talk about trying to make game controls more intuitive. The ideal situation is that a person with no history playing video games can pick up the controls quickly and easily, and retain that knowledge without need of a tutorial system.
Aside from ease of use, intuitive controls also engage players more strongly than more artificial control schemes. One of the main reasons I'm excited about the upcoming Tom Clancy's Endwar is that the use of voice commands in that game fits perfectly with the game's setting: a general commanding his troops from afar. Implemented well, a system of voice commands could really connect players to their role.
I was just thinking that spell-casting would be another intuitive use of voice-commands. In most fantasy stories, summoning magic involves some form of vocal incantation. What if players learned magic the same way as Harry Potter, Merlin, Allanon, or Ash ("Clatu! Verata! Nic...[cough]")?
I see two interesting consequences to this.
First, it would facilitate a natural and flexible learning curve. The player wouldn't be learning language like college students learn a second language (lots of foreign words all at once, and a bunch of mechanics which would be better acquired through intuition); the language would come just a word or phrase at a time, expecting little of the player. But there's flexibility in that a player could choose his or her own pace; by researching in libraries, meeting and learning from NPCs, learning from other players, or any number of other ways.
Also, voice-commands for spellcasting would add retention and recollection as gameplay factors. Player personalities would become a factor. One player might be the absent-minded apprentice who has to write down the spells on her hand because she's always forgetting. Another player might be the flustered bookworm who is always teaching spells to others, but whose mind tends to go blank when danger is imminent. Another might specialize in group incantations, able to carry on the lead incantation as the others repeat a different chant. And this also allows for some players to establish themselves in the community as truly phenomenal wizards... players whose encyclopedic knowledge and instant recollection makes them stand out in the crowd.
There's no denying that voice-commands as a game's only control scheme, as opposed to merely an option, is not conducive to a level playing-field. But a level playing-field is not necessary in all games; not even in all multiplayer games. Sometimes, compelling gameplay is best achieved by allowing for real differences.
Voice-commands could be combined with other intuitive systems for more depth and difficulty, if desired. Similar to D&D, spells could require a combination of voice and movement. Imagine having to draw a symbol with your mouse, ala Black & White, while also recalling a voice-command. It could be as simple or complex as you care to make it.