Not to endlessly chew it over, but obviously the fact that the background on the links has a gradient also play a part in the readability of those white coloured text links.
See the difference:
... which actually doesn't look bad at all. In fact, I think I might prefer a non-gradient not only because of the benefits of the readability of the links, but it also improves the look&feel of XF a little bit I think (for me). I feel the colour-sheme of the gradient and for example the background colour of the Members Online Now block makes XF (those areas) somehow look a little bit 'cheapish'. But it might be my monitor settings, so I am gonna experiment with that and obviously we have the Style Properties as well.
What I totally loved about for example vBulletin 3.8 was it's default style (apart from the remarkable out-of-place buttons that is). I never had the need to change it, because it looked and felt solid, strong, perfectly balanced. Very suitable for a corporate professional website, out of the box.
Ok I tried getting used to them, it just isn't happening. I think the effect takes away some of the cleanliness and simplicity of the former no-shadow navigation. I think Grover has a point: for more readability, increase contrast. I'll definitely use the style system to change this.
I find the links under a section, for example under Forums (Mark All Forums Read, Search Forums, etc...) a bit hard to read. But when I hover over them, they're fine.