Purely FWIW, the hype & release of the win8 dev prev. is a marketing push to try & avoid another hugely expensive game of Catch Up like MS is going through with Bing. They want into the cell & tablet market. It's not like it's going to cannibalize their Windows or Ofc sales, and with Microsoft's history of just abandoning projects & perceptions that they may not be the best company for devs to work with, they need a big marketing push.
Metro is a cell phone/tablet interface -- perhaps the main advantage of having it available on PCs/laptops is as part of that marketing push... publish in their store & you get access not just to cell & tablet owners, but every machine running win8. It *may* also help less PC literate folks use Windows, but there IMHO the jury's still out. It *may* help corp IT when/where they need/use touch screens like on the factory floor. It's not for everyday, normal PC use -- for that you have the normal Windows interface...
SO what else does the win7 SE have to offer? Any tech improvements they can manage to get ready by the time it's RTM. Better support for VMs, a more capable Windows' Explorer, the capability to run on less powerful hardware, *Maybe* Windows to Go [my guess is that it might be restricted to corp, and bring with it heavier DRM], along with other assorted bells & whistles like being able to open ISOs. Nothing more than a guess, I *think* that MS might be focusing more on special cases or situations they can sell to IT dept.s that have already moved to win7 -- convincing them to move the complete enterprise might be too much of a hard sell [Gartner seems to think so anyway].
PCs started as a hobbyist device [toy?], slowly moved into business use, & today are a large part of many, many everyday lives. PCs have also [& always] been very much Jacks of all Trades -- that's where things are changing nowadays... if all you want to do is watch movies &/or browse the web you simply don't want or need all those other capabilities, & certainly don't want to pay for them [in money, effort, or inconvenience]. Specialized PCs as appliances aren't just here, now, but they're being bought far more often than desktops. Cell phones, tablets, cable boxes, DVRs, TVs, eBook readers, hand-held players, cars, anything that can benefit from more intelligence is either becoming a PC or contains one. The general use PC isn't dead or dying -- it is being outnumbered. And if you can have a piece of every one of those sales, that's HUGE -- it may turn out that no one company can manage that, but it won't stop Microsoft, Google, Apple etc. from trying.