Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Design and themes have been a key part of the Linux buy-in for as long as I can remember.
In fact, once upon a time, the look and feel of application was actually used as its primary selling point to me. Back in 2000, at the LinuxWorld at Javitts, I made a comment about some app being very buggy (I don't recall which), to which the enthusiastic developer actually replied, "yes, but look at how cool it looks!"
Yes, well, DeLorians looked cool, too.
While I never thought looks should be the main reason you should try something--and thank $Deity my wife subscribes to that view, too--there's no denying that the design of Linux distributions and their component apps play a role in attracting users. We're human beings, with brains wired for visual stimulus-routines. Pretty->acquire is a very base reaction.
The folks at Canonical certainly understand that. I have watched with some interest as Canonical has put a lot of energy into the design and branding of its Ubuntu distribution over the years. I know that that founder Mark Shuttleworth has emphasized it more lately, pointing out the work his design and interface teams have done, Canonical has definitely highlighted the new Light theme coming out for Ubuntu 10.04 Desktop Edition.
Of course, branding with such ferocity can have its drawbacks. True story: a contact at Canonical sent me links to the new branding package, which I followed. I messaged back: "Huh. It's purple now." The reply: "It's eggplant. Actually aubergine."