Pre-8 OSes are definitely dragged along, with attention only shown to 7 just because it is their current OS and probably the one that people will be using for a long time (in business environment).
Gaming related, those developers that want to use XBOX LIVE for their games, will distribute the games through Windows Store in 8, and if they want to support 7, Vista and XP users, they will also have to distribute their games through Xbox.com / GfWL.
About what you said last, I think it will be a lot easier for developers to use both Windows and 360 for distribution of their AAA games since now XBOX LIVE is preinstalled (afaik, it came preinstalled in the CP) on all Windows installations, so users won't have to download other applications (like GfWL with XP / Vista / 7), and their Windows games will just work.
That is, if they decide to use XBOX LIVE, and not Origin / Steam / whatever for the Windows versions of their games.
I expect Windows Store to feature more indie and casual games in the first year or two, because most big publishers either have their own store or are using somebody else's store.
EDIT: I don't know whether what I wrote is 100% correct, since there really has been no info on how non-casual XBOX LIVE games will be distributed (none that I am aware of, that is).
EDIT2: What I think is freaking stupid is the fact that they don't allow C# and VB for developing games, yet they allow the abomination called JavaScript. Fuck!