Kaiser Shepard wrote...
Okay guys, let's stop this. Let's all be Jentha and get this topic back ontopic.
Personally, I think Krogan and Drell might also have a larger genetic variance than most other races, because Harbinger says they are also good enough for a Reaper.
Time to let my unfounded speculations run rampant again!

There is an interesting common factor between the Krogan, Drell and humans. All three royally screwed their home planets over. Humans survived because the Prothean ruins on Mars tipped them off about the Charon mass relay, and they could start colonies. Earth is still a polluted dump according to the Codex though. Drell survived because the Hanar helped them out. And the Krogan survived because they're just that badass.
Also, the three species vary across a certain spectrum, with humans in the middle. Krogans are big, strong and devastating in combat. Tons of strength and power, and a bit of cunning. Drell are on the other end, not insanely strong, but (if Thane is a reasonable indication) skilled and cunning. Humans seem to fall somewhere in between, with our highly fluid tactics in battle, and a nice balance between power and skill. We also just got lucky - developing space travel just in time to avoid the fate of the Drell, and coming from a relatively benign world (in comparison to Tuchanka, which is basically a planet of monsters) so that we weren't irrationally aggressive like the Krogan.
Maybe a certain propensity for violence interests the Reapers - Harbinger even says "aggression factor useful if controlled". Not sure why they dismissed the Turians then. Perhaps their highly-regimented species is too inflexible for their liking? Presumably they also have other criteria, since he also says they considered quarians because of their cybernetic augmentation, but dismissed them because of their useless immune systems.
I'm reminded of one of the interesting ideas from the novel
Blindsight, by Peter Watts. Concisely stated, the idea is that
technology implies belligerence. A species develops technological solutions because Nature gave them a raw deal, and so they fight back by controlling it. Someone living on a paradise world (like the ridiculous Avatar planet) would never bother to develop technology beyond some basic stuff. When there's enough stuff for everyone, there's no reason to fight either.
So in such a universe, the only species who make it out into space are the meanest, most aggressive bastards you can imagine. And in general, the mean ones are smarter than the nice ones, because intelligence gave them an edge in the constant battle in which they evolved. The Na'vi have no real use for calculus, for example. They're not going to build planes when they can just ride birds. And who needs computers and networks when you have a USB coming out of your head? <_<
I advise some caution when thinking about this deeply - if you totally grok it, it'll leave you depressed for a couple of days at least. I seem to recall an interview where the author said that when he worked out this and some related ideas, he fell into a sort of existential funk that he's never gotten out of.
Anyway, this isn't really a premise in the ME universe, but it's fun to look at it that way.