I’m sure others can provide
I’m sure others can provide more detailed information but I thought I would chime in 😉
1) With previous versions (v6 and before) the metadata was contained in the wiz.bin file I believe. So the key was to backup your config files on a regular basis. I haven’t made the move to v7 yet, but my understanding (which could be wrong) is that they changed this to incorporate the metadata into the recording files themselves.
2) When using extenders, no codecs are required. You just installed the SageTV server software on one system (in my case a plugin for WHS) and the extenders do all the playback functions. This is as close to the Media Center configuration as possible. If comparing apples to oranges (Media Center with xBox extenders vs SageTV with PC clients), then yes you have to install codecs on the client PCs to play back files. At least back in the XP days. I’ve never set up a SageTV client using Windows 7, which if it can natively playback media in Media player, it should be able to do the same with the SageTV client… i would assume 😉
3) Plugins used to be a hassle for SageTV prior to version 7. You are correct that you usually had to extract them to the proper folders, then configure settings. With v7 they introduced a plugin repository that could download and install them automatically. There are a few that still require making changes to a couple files, or making configuration changes in the GUI, but they made huge advancements in the process with v7.
4) For the base installation, I can get SageTV set up within 10 minutes from the time I start the installation. Customizing it might take a little longer, but that was one of the nice things about SageTV, it could be customized. Want a different interface? check. Want to include commercial detection/skipping? check. Want to add checking email to the interface? check. Home automation? check.
Media Center has cablecard, which is a huge advantage in it’s favor IMO. SageTV has recently been able to incorporate some cablecard via SageDCT, but it isn’t incorporated as well as Media Center. You are also limited to the copy-freely flags in SageTV. I’m really hoping the Google acquisition will alleviate some of that, but I don’t see it being on their radar since Google wants to provide their own TV, not an interface to watch TV from current cable/satallite providers.
Don’t get me wrong. I think Media Center appeals to the “mass market” much more than SageTV by themselves could ever hope for. To me, Media Center has always felt like a “gadget” added to Windows, and in the past has been treated sort of like the illigitimate son. Just look at the slow acceptance of Media Center into the common “living room”. It’s getting better with the xBox as an extender, but Microsoft just doesn’t seem to be pushing it that much. I know several people with xBox’s that didn’t even know they could stream media from another computer to their TV’s with it 
SageTV on the other hand has a “tool belt” feel in that you can add additional features/functions to it, which doing it’s primary job of recording TV. The occasional guest that comes over and looks at my SageTV setup give a “hmm, that’s interesting” impression and move on. Every babysitter/nanny that we’ve had (4 so far) that watched the kids every day during the week however have asked how I set it up and would love to have something similar.