Like I said, I always remember it being based on the same brand of scientific credibility as Live Free Or Die Hard, so it's not really ever been meant to be mentally taxing. That said, I only watched the older ones when I was about 6 and, as the show intended, I hid behind the sofa for most of the stuff between the intro music and the credits, so I don't know if they were deadly serious affairs. Who's really, really ancient and can recall whether the abysmal jokes are a post-Ecclestone thing?
The last episode was watchable. And.. who thought 'Bad Wilf' when he knocked on the glass door? I've a feeling he plays a far bigger part than just being Donna's Grandad.
That reminds me. I was looking for a mention of Bad Wolf when The Doctor went back to see Rose, but instead I saw an odd bit of graffiti looking similar - but different - to one of the Slatheen. Did anyone else see it? Any better identification? Edit: I've found some incomplete photos.