I can't speak for others, but I'll share why I like it. As always, YMMV.
1. The setting: it's obvious that Edwardian London was a locale that the production staff could pull off much more convincingly than, say, the war-shattered surface of an alien planet.
2. Leela: some nice costume changes, yes, but also great characterization. It was a story that used her background as part of the story, not just in throw-away lines. Though her defiance towards "Bentface" to the end was brilliant.
3. Jago and Lightfoot: for my money, the best of the Holmesian double acts. I particularly like the scene where Jago confesses that when it comes down to it, he's not so brave at all, and Lightfoot's saying "yeah well...that's the point, isn't it?"
4. World-building: There's a sense that this takes place in a big, big universe, with all those little details about Magnus Greel and World War 5 (?). All delivered with absolute conviction by Tom Baker, so you really believe what he's saying.
All things considered, for a 35 year-old tv show, it's got a lot going for it.