None of the stuff Microsoft is adding to the 360 is a particularly bad idea, but the implementation of most of it is kind of awful. For starters, the current 360 dashboard (the good ol' NXE) is getting more and more cluttered and it's getting harder to simply get to stuff on it. It's also a bad way to present features that should be more integrated. Like Netflix before it, all of the new stuff feels really tacked on and crammed in. 
 
Facebook: The Facebook stuff is good for looking at photos of your friends on your TV, and that's about it. The slideshow stuff is pretty good and it shows the captions. But the rest of it, which tries to use the NXE-style panes to show you different information, is a pretty poor way to convey things like your friend-based news feed. This thing should be custom-built for TV screens, not thrown together using existing Microsoft UI ideas. 
 
Twitter: Twitter is, at least, not just another NXE design with new data splashed onto it. It has a custom look that better supports what Twitter is supposed to look like. But the main screen is limited to the last 50 things your followed users have said, getting around to see your replies/mentions isn't plainly obvious, and the lack of a web browser prevents you from seeing what URLs your followed users are linking to. It's kind of a mess and I can't really envision an actual use for this. Anyone currently using Twitter is using a better interface for it (though I recommend TweetDeck if you aren't already using it), and anyone not using Twitter already probably won't have a very good experience if they're only using it on the 360. I really don't see the point beyond the obvious bullet point of "EVERYONE IS TALKING ABOUT TWITTER AND YOU CAN USE IT ON OUR GAME MACHINE." The whole thing stinks of suits.
 
Zune Video/Music Marketplace: The streaming tech for video downloads is a welcome change. I don't watch many movies, but since I've had the same two discs from Netflix sitting on my desk for close to a year now, I'm thinking about scaling that back to the lowest possible Blu-Ray-friendly plan, primarily using the 360's Netflix streaming for movies, and occasionally renting a movie via this new Zune-branded stuff. $6 or so for an HD movie isn't a great price when you consider the different Netflix packages, but if you rarely watch movies, it's probably cheaper to pay this on demand price than it is to pay a monthly fee for a service you barely use. It all makes me wish that Netflix had a streaming-only plan. 
 
Oh, and it's weird that these new stores launch you off into a separate application instead of letting you sift through content directly from the dashboard, and it's even weirder that the Music Marketplace is actually just a Video Marketplace for music videos. Paying $2 for an SD download of a music video feels like just about the dumbest thing in the world. Aren't music videos supposed to be promotional tools to get people to buy albums? Considering MTV has free streams of most music videos and YouTube has bootleg copies of plenty of stuff, I don't see the appeal. 
 
Lastly, the Zune branding doesn't make any sense. It comes from the same store, but it's not like you sync your Zune to a 360 or something now. It makes all of it feel like "Video Marketplace, brought to you by Zune." Maybe someday you'll be able to use a Zune Pass to actually listen to music.
 
last.fm: I mainly like last.fm from a data perspective. It's neat to see what I've been listening to broken down into charts and graphs. The streaming side of last.fm still feels like a poor man's Pandora, and the NXE-style content displays probably aren't the best way to convey last.fm's data. There also doesn't seem to be any sort of support for the social side of last.fm, like seeing other users, groups, and so on. Considering that most of this stuff also doesn't exist in last.fm's iPhone app, I can only assume that most people don't care about any of that stuff and only want to hear music. I've mainly used the site for the data and recommendations, but I'd rather sift through a lot of text and user profiles to get those recommendations and then make a decision to listen to specific tracks instead of just hitting some kind of "recommended for you" station. Oh, but the nice thing they've done is the video ads (which are mainly just Brutal Legend and a house ad for last.fm at this point) are the same volume as your music. So the ads don't blare at you to get your attention. 
 
So yeah, none of these ideas are awful, but I also feel like the 360 has already outgrown the "new" dashboard. They're trying to do too many different things and cramming all of it into the NXE's panes leads to a lot of confusing interactions. Also, I'm not really sure who the audience is for some of this stuff. Twitter especially sticks out as some kind of executive mandate to add to the Xbox 360 "story."