I believe with the older stuff they did a better job of knowing their limitations and working around them. ie having bad weather in parts of Jurassic Park, only using CGI to fill in fast paced, long distance shots in LotR.
Though I do love seeing some of the old movies that don't really stand up to being upscaled to HD too. Like in Resident Evil, those zombie dogs were suuuper creepy in SD, but now when you watch the BluRay it's quite obvious they just stuck some stuff onto a couple dogs
