With respect Wakawaka, your planned system is a hobbyists solution, as almost all other HTPC's are. AppleTV, and the other devices on the market like it, are an attempt to create a consumer solution to the 'problem'.
I'm not criticising your system, or other HTPC's, as they are obviously going to be massively capable, but they can also be massively complicated for the 'normal' person who just wants to watch "that video" ("codec? WTF is a co-deck?") on their PC, on their TV, without actually having their PC perched next to their Bluray player in their lounge.
The thing you have missed when looking at the AppleTV is it's simplicity. Joe Bloggs can sit on his sofa, browse iTunes on his iPad, buy a movie, browse his iTunes library using the remote app, hit play, and it appears on his big TV screen without once having to think "how do I get this to work?". As Big samm has said - it just works.
Bingo. That nails it. Not everyone is technically inclined nor do they care to put that much work into it. And unless you have the PC already set up to do the route that is being discussed, I defy anyone to show me the $99 PC you can get to cover the abilities of the Apple TV out of the box.