Here's the new home for the former iView Media Pro.
I haven't tried the current offering from Microsoft; I completely abandoned Media Pro when Lightroom shipped.
You are correct that Lightroom is not the same thing as Media Pro. Lightroom does a great job of photo library management for many (but not all) photographers. Lightroom is not a general purpose DAM.
I do not know what the long term prospects are for Media Pro/Expression Media at Microsoft. It's somewhat of an orphan for the Mac. (They've bundled the Windows version with a bunch of dubious graphics and web-development applications.)
In terms of alternatives,
Extensis Portfolio is still kicking around, although I never personally found that product sexy. Since you're on the Mac, you have the option of
Aperture, although that may pose the same limitations as Lightroom for you. I gather some people around here are simply relying on their file systems and Adobe Bridge (which I can't stand).
(Begin mini-rant...) Unfortunately, the development community seems to have decided that the Web is the best and only solution for digital asset management. Consequently, that's where all the effort is going: web servers. I believe these developers are—to a man—deeply misguided in their approach. A lot of us need a high quality DAM application that runs on the desktop, even if various forms of web integration is now an additional requirement. Both Extensis Portfolio and Media Pro/Expression Media are
old and showing their age. There is vast room for improvement.(...end mini-rant)
I hope something in the above is helpful to you.