Having been on the receiving end of some other build systems, I can say first-hand that a lot of them do an excellent job of making it easy to create a usable image that targets a particular architecture, say ARM, or particular application domains, say network routers.  But the more you stray from the narrow set of parameters those build systems are really optimized for, the more exponentially painful things become. Yocto makes the pain merely linear (and aims in the future to make it punctiliar, of course). ;-)

To me, things like the uPnP media demo are exciting because it really is easy to create both one-off and general-purpose applications like that.  How many times have I wanted to be able to pipe my favorite Barry Manilow tunes up the the party on the patio, while keeping the kids sedated in the basement with an endless loop of the Sponge Bob "F.U.N." song?  Well, now I can, just by going out and buying a few cheap boards and typing 'bitbake media-center-live ARM' or 'atom-pc', etc. for each of the boards making up my new home media center.  Or in the general-purpose sense, with a few tweaks, I could actually take that demo and make it the basis for a new business e.g. 'Tom's Home and Office Digital Infotainment Systems'.  It's always nice to have a backup plan, in case the current gig falls through, anyway... ;-)

But seriously, to the important stuff... It's always been a dream of mine to be able to apply real AI techniques to some robotics projects I have in mind, but until now, it hasn't really been feasible.  With cheap boards sporting e.g. powerful new Atom processors and a couple Gb of RAM in a tiny fan-less box, and an endlessly configurable build system like Yocto, some really interesting and non-trivial new robotics applications now seem possible.  For example, just to get started I'm planning on putting together a simple proof-of-concept Yocto application that, using only off-the-shelf parts, controls a simple toy robot using a USB-connected IR transceiver (parts below).  For 30$ worth of toys, a cheap Atom board, and Yocto, I can now create my own robotics distro for fun and profit...