I reckon I've found a solution to this - or at least I managed to resolve the issue you're all describing at my end; a new 2011 MacMini driving HDMI to a TV with a shocking picture quality (text with white halos, stark contrast, etc). This (unfavourably) compared to a Core 2 Duo Mini which had perfect picture quality.
So after trying
HDMI out -> TV = poor
Mini DisplayPort -> HDMI = poor
Mini DisplayPort -> VGA = great picture quality at a low res, but after clicking 'detect displays', avail res's changed and the picture took on a very purple hue...?!
The TV didn't have a DVI connector, so I couldn't try that path.
Just before throwing in the towel, I though I'd try coupling the MiniDisplayPort/T'bolt -> DVI adaptor with the 3rd party DVI -> HDMI adaptor we had connected to the previous MacMini.
To my great surprise and relief, this solution works! It makes sense too - the Mini now thinks it's connected to DVI and shows the full range of resolutions it previously showed with the previous Mini, rather than the TV-only res's it displays over HDMI.
SO, MiniDisplayPort to DVI, then DVI to HDMI and hopefully, champagne and caviar!