If they truly are going to take some effort in acquiring third-party games, they should talk to Sega (since they're out of the hardware biz, duh) and see if they can get some Master System/Genesis/32X/SCD/Saturn games as well. Like many of us here, I grew up with the full range of Nintendo consoles, but I was aware of some Sega games that were fun because of friends who took the Sega route or could afford both.
By securing the former competition, every gamer around twenty and up is sure to feel nostalgia (unless they owned a Turbo GrafX-16 or were strictly a computer gamer). However "niche" everyone thinks Nintendo is now, gamers from at least a couple generations back are very likely to have some fond memories of a Nintendo game, but it's not like there wasn't also a fair share of Sega fans. Getting old Sega games too would essentially give Nintendo a retro-monopoly of likely 95% of the game market.
I can't think of a Sega controller before the Dreamcast that would have more buttons than the N64, so if N64 games are playable, there's certainly more than enough buttons for any Sega games. I guess the problem there is when the games refer to the I and II buttons instead of A and B buttons (but if the controller can adapt to such controller inconsistencies as C buttons to C-stick or the right migration of the Z button, then maybe it could work).
But, sticking with just the past Nintendo consoles, there's a lot keeping it from a full selection. What about games from companies that don't exist any more? Japanese games? European games? Would other-region games be censored? (Ice Climber's seals or Devil World's title, etc.) Games that rely on specific controllers or devices? Other versions of games? Do you need a "Donkey Kong Classics" when Donkey Kong and Donkey Kong Jr. are likely to be available separately? Or like the version of Super Mario All-Stars that includes Super Mario World - would you even need Super Mario All-Stars without World or World on its own? Would we not see re-releases of games anymore? (Well, there goes the need for the button-swapped Mega Man Anniversary Collection...)
(And what about downloading Game Boy and Game Boy Color games to your DS? That would open up more issues - would you be able to get the 16-bit Space Invaders game that's only unlocked through the use of a Super Game Boy? Would Shantae or the Zelda Oracle games have the extra "Advance" things you get by playing them on a DS or would they be stuck on Game Boy Color mode?)
A lot of questions. I don't think it'd be likely at all to ever have a 100% complete selection. At least it's always possible that they could add more games to the download service over time.
In fact, if they do charge for downloads, they probably WOULD release them a few at a time. >_<
"Crystalis: relive the adventure in July 2007!" o_0
I wonder if they can hype up the re-release of games that are downloads at least a fraction of what they do for new games.
I've rambled, but seeing as how people seem to like those controller-devices with emulated classic games on them, even if the Revolution's download system was just region-specific Nintendo-made games for a small fee, people would still buy (or rent?) them up like crazy. I know I would.