What concepts does Rust introduce that aren't already present in the latest C++ standard? Which aren't already present in Scala? Which aren't already present in Go? Do you really want me to believe that memory safety, concurrency, generics, and exception handling present in half-a-dozen off-the-shelf (and mature) languages weren't sufficient? (And let's not forget that every new language departs from existing tool chains that service to multiply development efforts.)
I'll say it for the umpteenth time on Slashdot: why do we need dozens of half-baked projects instead of a few that are exceptionally solid? And I'll, again, give my own answer: because every one of these geeks who designs to start his own little, duplicative project thinks he's smarter than everyone else. You know, because those guys are idiots, and we couldn't possibly build upon their work. Greenfield development is always fun! (Am I right?)
But let's forget all that for the moment, and concentrate on our shiny new browser engine. If and only if this project is successful when all is said and done, it will produce exactly the same output as its siblings and predecessors--output that's dictated by standards. Indeed, after devoting person-years worth of hard labor, we will end up with something with exactly the same functionality as what we had before.
Sigh. Obligatory XKCD.
http://xkcd.com/927/