A lot of UK MCs have pretty shit voices, they sound whiny or anonymous, or anonymously whiny. This is the case with a lot of Road Rap that I've heard - the beats are good, the lyrics are alright, there might be some nice punchlines in there etc. but the MCs voice is just a bit boring.
I almost get the sense that, whereas in grime MCs accentuated aggression/idiosyncrasy of tone/delivery, in Road Rap there seems to be a conscious effort to drain delivery of emotion - maybe to project the image of cold hearted badman or something? Or maybe just because it fits in with the production. Actually I think this contributes to the overall sound and atmosphere of the music (I.E. Roots - Cruddy Freestyle is entirely generic but I love it), but it also makes it seem a bit dreary to me. Giggs has that imposing deep voice that makes him stick out within the scene (although I listened to his latest mixtape the other day and his voice bored me after a few tracks too).
American hip-hop seems to have a lot more range in vocal/production styles, but then it would, since it comes from such a huge range of places within a gigantic country, while Road Rap I suppose mostly comes from London (and probably from a few small areas within London). Grime MCs seemed to have that range of delivery/voice/style, though - Wiley, D Double, Goodz, Trim etc. are all instantly recognisable as well as being technically brilliant. To me Grime is/was the real UK Hip-Hop; it wasn't a knock-off of an American original. I can't dismiss road rap as being that (although it seems clearly more in thrall to American hip-hop than grime was) cos I haven't heard enough.
I love UK MCs in jungle, garage, funky and grime, though. And some UK Hip-Hop MCs are good too - Roots Manuva, Kalashnikov... But yeah, I can't really get past this notion that UK Hip-Hop is essentially a lesser version of American hip-hop.