I disagree that the choice is between ruining the game for the majority of people or ruining it for a minority. I think that it is a choice between forcing everyone to have to deal with a minor inconvenience, or completely ruining the game for a minority of people.
Honestly, I don't think it's a horrible fate, to sometimes have an early leaver in your game. In fact, it has been my experience that people almost never leave the game unless the game is already decided. I shouldn't be horrified at this thought: the biggest problem is that the game currently freezes for thirty seconds while the player disconnects, then if other players follow, the game continues to freeze over and over, then you're left fighting a bunch of computer players. The concede option would fix all of this. Keep in mind too that this game is very prone to crashes. But putting in a karma system isn't going to change that.
What I don't see often is players getting killed a few times early on, and rage-quitting before the game is actually decided. Is this just me?
Regardless, a big problem with DotA is that people boot others out of the lobby for, in my opinion, absolutely no good reason. A ping too high maybe, or maybe just a whim. And I'm already starting to see this happen in Demigod. Even with just ping, I can't count the number of times I've seen players get booted because their ping is too high (even though I've seen players with higher ping run just fine). The last thing I'd want to do is give game-creators even more incentive to boot people joining their games, and this is exactly what these sorts of karma systems do, except that the karma system is worse because once your karma is low, you can never repair it.
As for ESEA, I personally think their karma system works horribly and its problems are masked because only the most hardcore of players play EA Sports games online.
Even if karma is implemented, I don't expect I'd have a low karma. But if I somehow did, you can bet I'd be asking to refund a now-unplayable game.
It's either just you, or it's DG in general. In Dota, people leaving after being killed once or twice early on or even leaving because their favourite hero got picked or they didn't like the hero they got given are commonplace in my games. Admittedly the problem isn't as bad in DG, but considering how much the AI feeds in this game, a leaver early on could potentially ruin the game (That is, unless you have AI switched off). You say it's a minor inconvenience now (And in DG yes it is), but if there were somehow a mass migration of Dota users to DG, the number of asshats would shoot up, and it would cease to be such a minor inconvenience. Oh and also, another thing I just thought of, whilst a concede button would indeed help, on Dota there ARE people who will manually CTRL+ALT+DEL and kill the game process, which is the same as a disconnect, just to piss people off.
Also, never assume that a low karma couldn't be fixed. Like spazznd said above, having the karma calculated from the ratings given over the last 30 days would probably give a good indication, as well as allow people who started off sucking miserably to recover once they improve. Besides, a karma system like that (I really must stop calling it karma though, it's more like player statistics what i'm describing) would be very useful in helping the matchmaker service give good matchups ie: Let the nice people who play well, don't flame, don't ragequit, etc, play against each other, noobs can actually go up against noobs (Or, even better, a sort of parenting up mode where a pro captain teaches a noob team how to play), and the flamers can flame away to their hearts content with other flamers, etc.
Oh and one final thing, actually following those player statistics would be entirely up to the host. I'm sure for every host that wants a decent game and would want to filter out flamers, ragers, etc, there's a host who really wouldn't give a damn and let people play anyway.