FYI, the Blu-ray tier thread at AVSForums is by no means gospel truth; it is generally based around the recommendation of one or two people, and is the subject of a great deal of controversy within that forum. It's a good little quicklist sort of guide, and nothing is grossly out of place (ie. Gangs of New York isn't Tier 0), but it's still taken from a relatively small sample size (especially movies without broad mainstream appeal), so take it with a grain of salt.
A great resource for looking at the picture quality of Blu-ray versus DVD are the threads posted by Xylon at those same forums. He does timestamp comparisons between DVD and Blu-ray, and often HD-DVD and broadcast HD as well, and highlights any anomalies such as DNR, EE, framing errors, etc. (It was his thread on the framing errors in Pirates of the Caribbean that prompted Disney to reissue the movie). You can also visit Blu-ray review sites like highdefdigest.com for reviews that discuss picture quality specifically.