Even with the new rendering engine in Lightroom 3, DPP does a better job rendering highlights.
I would love to have an improved rendering engine in future versions of Lightroom.
I've never been sure how much of this rendering is due to engine and how much is due to camera profile (improved D300 camera profiles have improved highlight rendering), but I agree: highlight rendering is very important, and Lightroom does not always lead the pack in this regard, at least not for all cameras.
Also, I'd love to see a general purpose gradient smoothening tool that one could lay down to get smoother gradients, for things like sunsets and concert lights... So one could have better transitions between completely blown out to partially blown, to not blown out but still sparse color info... (and not just transitions, but also being able to have the completely blown out regions assume a color other than white/grey). And not just for highlights, but could be used in shadows too, after uping exposure of a high-ISO shot, as example, to smooth shadow colors. Or even in midtones I guess.
In other words, part of the problem is in Lightroom (how highlights are handled), and part of the problem is in the camera (sparse info at the endpoints of dynamic range), yet the solution will need to be all in Lightroom (assuming no control of camera manufacturers - things like built-in HDR will help...).
Clarity doesn't help for extreme highlights (5D MKII). The gradients still look artificial and not smooth.
I also don't want a general purpose gradient smoothening tool, I want the highlights to look smooth and realistic out of the box (i.e. immedialty after importing the photos into Lightroom). It would be too much work to manually adjust different highlights from different photos.