I'm confused. Do you mean what advantages Windows Phone 8 has over Windows Phone 7?
In that case irlju has it pretty well covered so far, but in terms of the things you're asking about; messaging and people hub and so on? I think it's fair to expect that there will be some minor changes to the social stuff, both on the Phone OS and the Desktop/Tablet version to bring them more in line with each other. But at the moment the WP7 implementation is a long way ahead of what is currently present in Windows 8 (desktop), so there's not really anything that could be inferred from that. If anything further functionality has to be added to the desktop/tablet version of the OS.
Multitasking I can't see changing a whole lot on the phone outside of the way that will operate on a base level. Certainly I don't think snap multitasking will be added; it works ok on a 1080p - 27in screen, but no matter the resolution, it's gonna look stupid on a 3-5in display.
I'm not sure how helpful any of that is because you seem to be asking about OS features that are of more relevance to consumers rather than developers and at this stage pretty much nothing has been said about that. Although built in skype support will be there.