The convergence of TV and the internet is an idea that has only really taken off on the torrent sites, but has a lot of potential, especially if someone can…
1) Apple TV/Xbox 360
These both have pretty much the same story to them: you buy content from the respective stores, through the device, then watch it on your TV. Apple TV can transfer content from your iTunes library, but the Xbox is restricted to either Zune purchases or streaming. Neither offer any kind of subscription video service, nor do they interface well with existing content distribution services, eg cable TV, BBC iPlayer, Youtube, etc. Some of this is the fault of the providers (cable) and some the fault of the manufacturers (iPlayer, Youtube).
2) Boxee/Google TV
I currently use Boxee as my video manager, and while it can handle local files fairly well, it's pretty sucky at streaming. I live in the UK, and most of the online content I want to watch, eg Revision3, etc, it hosted in the US, and I want to watch it at 7pm, which is when my ISPs network throttling is in full swing, rendering streaming anything but the lowest possible quality video completely unwatchable. The lack of a download client of any description is quite crippling in my mind. The fact that the software is also unfinished and still fairly buggy is also a big cross next to it name. Google TV looks to be going the same route as Boxee, but at this stage it's too early to say for sure how dis(similar) the two really are. Boxee has many third party apps available, which is something Google will need to look into.
3) Home Theatre PC
The traditional option, but not the easiest to set up and even harder to maintain, especially if you're going to be using multiple applications, at which point you're going to need to pull out a keyboard and mouse. I've been considering this option for a while now, and with the Acer Aspire Revo costing only £240 with Windows 7, it's a good option considering the alternatives. I don't own a TV so all my media comes directly from the internet, and Windows is versatile enough to suit all my whimsical needs. the problem is that there's no one program that completely satisfies me. Boxee, as I described above, still needs work. iTunes, as has been mentioned elsewhere on the site, is an abomination, as is Zune. Miro's pretty cool, but is fairly limited in terms of functionality. Windows Media Centre is good for local files and live TV, but has next to no third party apps available.
The way I see it, no one has been able to really corner this part of the market yet. Most devices/services so some things well, and fall down on the rest. With Google being an internet company, they're well placed to bring the internet to the TV, especially with the help of Sony and Logitech, and as an advertising company, they're well placed to strike up relationships with content providers that could see prime time shows hitting the internet. Then again, they also produced Buzz and Wave, and we all know what powerhouse successes they've been.
In regards to the Home Theatre PC, it's all dependent what your needs are. Boxee & Plex are the most complete package for most people so a lot tend to go to that, however it's extremely limited in working with LiveTV. XBMC doesn't work with LiveTV the way WMC does but it does work with HDHomerun which essentially gives it the ability to watch live & recorded tv via UPnP. WMC is the most obvious choice for the average consumer because it's pre-loaded into windows 7, works with live tv and can function with any xbox 360 in the house; despite what you say, there are plug-ins for WMC and I don't know how much you're looking for but they do have R3 & Netflix.
I get a lot of your negatives are but when I was putting my HTPC together, simple searches yielded me a lot of results and answers. As it stands now there is no defacto media center software that does it all because there is always going to be limitation. So thats why I said you need to find the one that fits your needs best, you may need to sacrifice convenience for better overall performance.
I originally ran Boxee but found it clunky & I didn't like how it centered around feeds, so I moved to XBMC. By doing this I lost direct apps for Revision 3 & MLB.TV but I had alternatives. To address Revision 3 I just set up Miro and let it run idle in the systray and have the download directory monitored in XBMC. As for MLB.TV, well that is now a more full featured app on my PS3 so I just turn that on when I need to.
Download and Install Miro
You can add in all the shows you want to watch on Revision3 and just set Boxee to monitor that directory. Miro will just tuck away in your system try and download the episodes as they become available.