I think there are a few reasons for this. One issue is that technology has improved a bit in the intervening years, so it's unfair to expect old programs/films to be up to modern standards. However, I'm not doing a direct comparison; for instance, even before I saw the new Narnia films, I still remembered the cartoon being less jerky.
Another issue is that TVs have improved too. Paul O'Brien made an interesting observation about high-def TV for wrestling:
Although they've been singing HD's praises publicly, the WWE are privately rather ambivalent about the technology. They certainly accept it as the way things are going. But professional wrestling is the art of not hurting your opponent, as convincingly as possible. It's designed to be viewed from halfway back in the stalls - preferably through a smoky haze by somebody who left their glasses at home. It's not entirely TV-friendly to start with.
High definition pictures only make it easier to see lack of contact - not to mention other embarrassing features such as mysterious forehead scarring. Apparently, there's also an increase in sound quality which makes it easier for the audience to overhear wresters improvising by "calling spots" in the ring.
I don't have HD here, but the TV I have now is bigger than the one I watched as a kid, so I may get a clearer view that's a bit less forgiving.
However, I think the main reason is simply the passage of time: it's been so long since I last saw these programs that I'm having to fill in the blanks for myself, and I do that based on what I'd expect to see. I've probably also been influenced by the novels, which described the way that all the creatures were supposed to look (e.g. Sontarans being slimy, Aslan being a mighty lion).
I think this is linked to the way that our minds work, to do with "chunks" of information. There's a similar concept for writing: if I'm copying down a word in English, I'll do it in my own handwriting, because I'm treating each word/letter as an abstract Platonic ideal. In other words, I'll read it, then in my head I'll just remember the words, not the specific font design, and I'll write them the same way that I normally would. If the original text had serifs (tails) on letters, and my version doesn't, that doesn't matter; it's still the same information. By contrast, if I need to copy down a word in Cyrillic (the Russian alphabet), I don't know which particular penstrokes are significant, so I'll try to mimic the original writing as closely as I can. The snag is that this takes a lot longer, since I'm treating the letters as pictures (devoid of any semantics), and I'd have trouble remembering an entire sentence of unfamiliar glyphs.
So, coming back to TV/films, I'll just keep a mental template of what a Sontaran looks like, drawn from various sources and updated as I get new information, then I tend to retroactively apply this to any previous stories.
On the flipside, there are some cases where I can watch an old program and get more out of it, because I notice things which I was oblivious to the first time around. For instance, I've been watching re-runs of The Good Life, and I'm impressed at how well it holds up: the characters are believable as married couples, and I can empathise much more with someone who is made redundant. As for Dr Who, I was 10 years old when I first saw that episode, so I never noticed Peri's cleavage, but it's fairly obvious to me now!
Meanwhile, if you can't get enough of my blog posts (or think that these are too long to read), I now have a microblog at Twitter. Each post is the equivalent of a text message, so it's a good place for pithy observations; either that or minutiae like "Today I ate a biscuit".