There's been a lot of debate in the Grandma's House thread about what is and isn't a sitcom. I decided to open a thread here, so all the IT'S COMEDY DRAMA/IT'S SITCOM furore could take place here and not denigrate the chat about Simon Amstell and his wonderful show.
Personally, if it's comedy with a narrative and 30 minutes long, I class it as sitcom. Obviously they can take many forms though in that timeframe. The Office and My Family are both very different in things like the execution of comic dialogue and production, but they're both classed as sitcoms.