DotEmu could definitely shape up to be a serious contender, and as I pointed out somewhere else a few days ago, they're expanding their catalogue at a faster rate than GOG are (which is fairly natural, of course, as they need to catch up, and I bet many companies are easier to negotiate with after GOG has shown them that this business model works).
In my opinion it's good news that we get a competitor for GOG. It'll keep GOG on their toes... or so I hope. Right now GOG doesn't really seem to be responding to the competition, what with them only releasing one game per week (which is something they need to change soon).
But it could of course be that the reason for GOG's low amount of new releases is that they're preparing something behind the scenes. I mean, if we were to speculate wildly, it could be that they're finally going to start a section for consoles/other systems. And if they, for instance, were to launch SEGA's Mega Drive games here, it wouldn't make sense to launch them one by one when they're available on Steam and other channels already. So maybe they're silently adding a bunch of games so they have a selection ready once they launch the new service (on GOG's second aniversary, maybe? :-) ). Well, that's what I'm hoping anyway.