It would be played with a computer, at a table, with real-life friends. Each player would use his or her own touch-screen phone or a laptop. The owner of D&D would produce different versions of the D&D software compatible with different phones and OSes. In a pinch, players could have multiple PCs on the same phone, but I figure most everybody will have a smartphone in 10 years.
Each player has the stats for his PC entirely in the phone, and also stored on a distributed database so you can download your character onto other folks' phones, or onto your computer to fiddle with it.
The GM probably has a laptop (or touchdesk) setup, where he can manage encounters. In combat, the map is streamed to each player's phone, and if you've got one you can also stream to a central tabletop screen, or a TV. You can move around with a few quick touches.
The GM has a turn-tracker, and he can tap on a PC's icon to show whose turn it is, which alerts the player. If you're in the 'beginner mode' for folks who haven't played much, that will cause a menu to bounce at the edge of the screen; open it, and it gives you reminders of what to do this turn, like:
Do these actions in any order on your turn.
* Draw a weapon (or take another minor action - examples).
* Move X squares (or take another move action - examples).
* Make an attack (or take another standard action - examples).
Suggestions.
* There's a group of goblins bunched together. Your icy burst could hit 4 of them.
* The goblin chieftain is next to your bloodied ally Kevin. You could use confusion to get him away from Kevin.
* You're a wizard. Come up with something magical (but not too crazy; remember you're just 1st level), and ask your DM whether you can do it.
The key is, this isn't a video game; it's a computer-assisted tabletop RPG. You still roll dice, add the numbers in your head, and your GM adjudicates weird stuff not in the rules. When you use a limited-use power, the program on your phone makes it easy to mark it as expended.
I repeat. I love rolling dice. I don't want a tabletop game where computers roll the dice for me. Of course, it will probably be an option in the game, especially for ease of demo'ing.
When you roll damage dice, you just select your character, tap the 'damage' button, and tap the amount of HP lost. Likewise, the GM ctrl-clicks each injured enemy and marks how much each took from an area blast. Hell, there'd probably even be an option that lets you tap on the grid, drag out to a burst or blast radius, and then record "full damage for these guys *tap tap tap* and half damage for these guys *tap tap tap*."
If you need to look something up on the fly, an intuitive index (and bookmarked pages) let you find what you need in a hurry. Imagine if you could just do a Google live search for "Page 42," or "Lightning Bolt," or "Nycaloth." (No more having to go to your computer, log into the Compendium, use a clunky interface, and print out stats.)
Of course, if you want to run a game for your friends who have never played before, everyone installs the free demo of the PC tracker app to their phone, then selects from a set of pre-genned characters. You all follow through a 5-minute tutorial as it walks you through the basic rules, and boom, you're ready to start.
Oh, you were curious how I'd change the rules? Hm. I dunno. I haven't thought about it much.