HE wasn't actually that funny in the office, some lines were great, but not always his delivery. But it was the ackwardness and abnormalness of the behaviour that was funny.
However after watching the American office which is genuinely, much funnier with far far better jokes, when you rewatch an english ep, you realise its really just uncomftable humour, not really classically funny.
I don't like his stand up or much of what he's done, thought Extra's was dull, dull writing, dull acting, he wasn't really funny and he wasn't ackward.
His last film, where he's a dentist, forgotten the film. Its pretty standard, predictable but still nicely done with some decent stuff, but again he's kind of that mean hearted git and for instance the bit where he's signing into the hospital, its really him being ackward and strange rather than actually funny.
I think he plays strange and ackward very well, I don't think he's close to a great comedian and anything he's been in isn't really funny start to finish, just a few good one liners spread throughout whatever ep/film he's in.
He's ok, and seems a nice enough bloke in real life but just not that funny. New, different to the norm and alright in small doses.