Personally, I have no idea what to make of this.
Firstly, I don't agree with the notion that Derek is necessarily disabled. Some people just are that little bit ... well, thick, without actually having any kind of learning disability. But whatever he is or isn't supposed to be, I agree with sooty's argument that we saw a load of gestures and mannerisms with only limited amounts of the actual person.
As far as the comedy goes, it got one small (but admittedly, reasonably satisfied) laugh from me; Dougie's reaction to Derek running naked through the lounge area.
I'd also hoped that the mock-doc elements in the trailers were created for the trailers, and we'd get some kind of variation in Gervais's range. No such luck. Same boring, tired old tosh that he produced MORE THAN A DECADE AGO. Frankly I find it depressing that it's still being made.
However, there was a nice bit of pathos surrounding Joan's death. It did feel a tad overdone, and I didn't quite buy into Derek being so devastated by it: he's supposed to work in a nursing home, and he's still so gravely personally affected by the death of a patient? Pull the other one. But I guess it serviced the plot...
Further on the realism of the show, it was a great pity that they crowbarred in the wake scene. That just does not happen in any care home I've ever heard of. People are almost spirited away without further mention when they die; there's certainly no black dress event.
Having said that, it wasn't absolutely awful. The story was sort of interesting, although certainly not gripping.
However, I think it'd be a real mistake were Channel 4 to order a series. It was a curious little one-off but didn't sow enough seeds of promise, and I'm struggling to see where they could take the characters that wouldn't end up being an inarguable mocking of the disabled, the infirm and the elderly.
They've always been known for pushing boundaries with their programming, but that kind of mockery would be a bit too risqué even by Channel 4's standards.