I think it's only a display artifact in the blending of the transparency grid (the checker pattern) with your layer. The brush mark doesn't have that problem when there's an opaque layer underneath the transparent layer. By the way, I recommend you work in 16-bit mode to reduce posterization and banding when multiple semi-transparent layers are stacked, or semi-opaque brushmarks or gradients are overlayed. 16-bit mode reduces posterization and banding in adjustment layers, too. Converting from 16-bit to 8-bit when required will dither by default.
Thanks for the answer, switching to 16bit did it!
Still a bit irritated about why in CS4 everything worked just fine in 8bit... guess I'll just have to update all my documents to 16bit then.
Working on opaque backgrounds definitely helps as well, but I work a lot with semi-transparent graphics and stacked layers!
I think there's a slight misunderstanding. I do recommend 16-bit, and 16-bit indeed does not have that display problem shown above, but your 8-bit work in CS6 is not inferior to 8-bit work in CS4. As I said, the problem you showed is only a display artifact - it isn't a part of your files.
Anyway, since your work involves stacked semi-transparent layers, you should work in 16-bit mode from now on.
Something about 16-bit mode to be aware of is that at zoom levels less than about 65% when Cache Levels (in Preferences > Performance) is greater than 1, the display is calculated with reduced precision, so 16-bit mode might look no better than 8-bit while zoomed out to below about 65%.
Also realize that your OS, graphics driver, graphics card and monitor will be incapable of delivering a 16-bit output from the screen, so don't be surprised to see some banding on the screen regardless of the bit-depth you work in. A 10-bit per channel display pipeline will improve over standard equipment. If you're interested in that, there have been discussions in this forum.
I haven't seen anything I would call "worked just fine in CS4" that doesn't work as well in Photoshop CS6.
If you can specifically show that the exact same operation in Photoshop CS6 is producing worse image quality than when in Photoshop CS4, I imagine Adobe would be quite interested.
I've seen similar artefacts using PSCS5 on an iMac. Turned out to be caused by the settings in prefs > performance > Graphics Processor Setting > Advanced.
Set it in a less advanced mode.
That means that your video card or driver has some problems.
We have seen quantization problems with some video card drivers (failing to execute the shaders correctly, and losing precision).