Northern_Brewer
British - apparently some US company stole my name
Aside from the lower carbonation and natural carb/conditioning (which may or may not make a flavor difference vs force carb)
Oh it does make a difference - you get a finer carbonation that is less "intrusive" and more persistent (compare the flabby big bubbles in Coke with the fine mousse of naturally-conditioned champagne). But it's not just the bubble size and fizziness, perhaps the main problem with kegging British styles is that the actual taste of the carbonic acid from excess carbonation ruins the delicate balance of the beer. Nitro helps with some of that but doesn't solve it.
The very slight oxidation you get in cask can also be beneficial. It tends to meld the flavours together to form a harmonious whole. Sometimes you don't want that, in the same way that a fruit salad benefits from being able to taste the burst of a grape or the discrete taste of a pineapple chunk, but for traditional bitter you generally do. Oxidation can knock a bit of a flat spot in beers that depend heavily on modern fruit-flavoured hops, it really depends on the beer. One of the nice things about the modern beer scene in Britain is that you occasionally get the chance to taste the same beer on keg and cask (and sometimes on bottle and/or can) - and I can safely say that I've had beers that were best suited to each of the four main formats - and it's not always the one you think it will be!
As has been mentioned, sparklers are a regional thing and really need beers to be brewed specifically for them, to allow for some of the flavour being knocked out by the sparkler. But even up north you'll find CAMRA festivals serving cask on gravity, which obviously requires a lot less plumbing and is something to perhaps consider, particularly since North American brewers are convinced that bitter has 5+% crystal which is far more of a southern thing. If you insist on making bitter with >5% crystal, you don't need a sparkler.