Why not a PC "front end"? Because they tend to look Heath Robinson at best in an AV set-up, the user interface can more often than not suck, it's overly complicated when you simply want to stream iTunes stuff...
The new apple TV is simple, elegant, cheap which ties in seamlessly with my iTunes/iPad/iPhone without having to mess around with settings, network set up or any other 'computer stuff'. It's the 'computer stuff' which has put normal people off HTPC's (which cost LOTS more than an AppleTV)- the AppleTV is that plug'n'play solution. I have an old Sony laptop with a broken screen, tucked out of the way which I use as my iTunes 'server' - the AppleTV is simply the front end.
The lack of 1080 is a total red herring as far as i'm concerned; the only 1080 media I have is on physical bluray disc. HD TV is 720, everyone still watches most of their TV in standard definition, HD film rentals on my cable box are 720, most online sources for legitimate downloads are 720 - even if they claim 1080 resolution the compression usually makes them worse than true bluray quality.
The Roku boxes look pretty cool actually, but many of the features are unusable outside of the US, and most importantly it won't access my iTunes library, so not really suitable for me. I do think it's a good option for those who aren't Apple fans though.