First, I think people are going overboard, it won't look exactly like that concept car. It will take cues. You know, like the new Fusion took cues from that similar design style. I'd expect the back end to be sleeker than the concept for instance - more fastback and less hatchback).
Personally, I think the Mustang has always looked ok, sometimes great, but never what I'd call the pinnacle of what it could look, so I'm all for a more futuristic look, and think it makes a lot more sense for the Mustang. They still could offer a more retro look as a package too (something that I think has set the current Mustang apart very well, as its easy to get a unique Mustang due to all of the options and packages available).
To give you an idea of how different it can look, go look at the Giugario Mustang concept from a few years back and compare it to the current car which used it as inspiration for. There's still a pretty marked difference between them although you can certainly tell the cues it used.
Also for those bitching about the "more European" you need to consider context. Think more in line with Cadillac's change with the CTS (and GM and Chrysler in general, both of which have done a lot better by applying some Euro finishing to their lineups). I expect they mean they'll just eschew a lot of the retro styling cues for a lot of the newer cliched stuff (that more aggressive-strained grille and headlights with more of the styling creases in the sheetmetal, more technophile gauges/controls, and lastly I would take that as a pointer to them finally moving to IRS). Basically, think of a Ford version of the new Camaro (which is actually a big hit despite being an outright worse car than the Mustang, although its not crap, the Mustang is just possibly the best its ever been).