Okay, so a certain amount of belief has to be suspended to enjoy this, but that doesn't mean it's not good TV. There seem to be far too many comparisons to the West Wing. The Palace is basically a tongue-in-cheek dramatisation of a whole bunch of 'what-ifs'. And, if you can get over the initial premise and look a little deeper, there are quite a number of points here about male succession, the spilt between crown and parliament, staff loyalty, the duty of monarchy and so on. This has not just been thrown together - in fact by making all the characters fictional the writers have much greater license to look at and expose some of the more interesting facets of the UK constitution.
The main problem is that it falls between two stools. It's too silly for a Royalist drama, and not quite biting enough for black comedy. It would actually be much better off on Channel 4. ITV doesn't have much of a history of 'faintly subversive' so doesn't know what to do with it, and the critics don't know what to make of it.