The Dark Eldar don't really bother me that much. Eldar nature was to take everything to the extreme, prior to the Fall. The Dark Eldar are a reflection of that, coupled with the high tech and typical Eldar arrogance that is perfectly happy to sacrafice a million lesser lives to save one Eldar one.
The Tau I think do fit in the 40k universe .. it is a big universe after all. And while they may have started off fairly innocent, running into the Imperium, the Dark Eldar, Chaos, the Tyranids, that's going to open their eyes in a big way. Does that mean they may start using nastier tactics? Sure. From their perspective, The Greater Good can sometimes demand harsh sacrafices, and the loss of a few individuals is necessary for the whole to prosper.
I think perhaps though the Squats could have been resurrected instead of bringing in the Tau. Or perhaps in conjunction with them. You have the high technology angle without the mysticism that the Imperium invests in it.
Speaking of which, anyone else been seeing the rumours that we might have an AdMech army coming out for 40k?