Yeah, that’s pretty much the story. Vidoe taped series TV shows up to the mid 90’s were shot on 525 line media, and they’re not going to get any better. Filmed shows were usually transferred to SD tape for post. One exception is those shows that were sold to European syndication, where the original negative was cut subsequent to the US airing. A re-transfer off those negatives is pretty attractive.
Studios started converting movies to DVD back in the mid 90’s, and all those transfers were 525 SD, so there’s a lot of that product sitting around, and an upconversion from those tapes is pretty lame. You have to find a Blu-ray disk where they actually retransferred the film in HD in order to really get the quality improvement.
Resolution is the biggest part of the picture, but don't neglect frame rate. Film masters are 24 fps, NTSC video tape 29.97 fps, material originally shot in HD 60 fps. That's why HD is so noticeable in sports or in action flics -- depending on the source and what's going on on screen, 720p at 60fps can look better than 1080p at 30 or 24 fps.
Upconverting and deinterlacing video involves a computer making its best guess at what comes between frames or between lines. It's like blowing up a picture to a higher resolution in Photoshop. A good algorithm is better than a bad one, but the best algorithm is a poor substitute for quality source material.
I recall hearing in the early days of HD broadcasting that TV Land was one of the first networks to have a substantial HD library. Network shows of the last several years were shot straight to tape, while the older shows had film to go back and scan.
As for BD, things like Planet Earth are a must in BD. I buy my TV series like X-Files and Stargate in DVD but I would buy the LoTR Trilogy in BD as well as anything Pixar.