Cash Cow Franchise: As well as its 42 year run, the TOTP format was sold worldwide and spawned a magazine in the UK (which is still published despite the end of the weekly series) and many compilation albums (the 1970s ones not sung by the original artists, the 1990s ones actually sung by the original artists).
Channel Hop: from BBC One to BBC Two in 2005, what turned out to be the final nail in the coffin for the weekly series.
Keep Circulating the Tapes: Since the Savile sex abuse scandal broke, the BBC has announced it will no longer show reruns of any of his TOTP episodes, rendering a substantial chunk of the surviving run this. Going forward, the only place to watch footage of Savile-era episodes is likely those uploaded to video-sharing services (including those uploaded prior to the ITV-aired exposť).
Missing Episode: Most early episodes of TOTP were wiped by The BBC; only four complete episodes exist from the 1960s (one and most of another with the presenter's links mute), and the show's archive only exists in full from 1977 onwards. And as mentioned above, much of that is now banned.
No Export for You: Aside from an extremely brief run on BBC America in 2002, the only episode to air on American television was the last one, which was aired on VH-1 Classic in August 2006.
Too Soon: The BBC's reasoning behind dropping all episodes featuring Savile and/or Dave Lee Travis (who was arrested as part of the Scotland Yard investigation of Savile's crimes) from the BBC Four rerun rotation. The question is whether it will ever stop being too soon.