Every time I've picked up someone else's digital camera I've needed a quick walkthrough of how to shoot a picture with it -- but the fact that there's a lens, a viewfinder, and a shutter button generally doesn't change. Likewise, every time I've sat down to work with a new Linux distribution, I've had to re-accommodate myself to a different desktop or package manager, but the command line and the kernel are just about always the same.
Odds are there are enough similarities between this particular camera and almost all the others out there that there won't be much of a learning curve, but I still might need some hand-holding. Do I have to manually open the shutter? How does the autofocus work? Is there even an autofocus, or is it one of those cameras that's one meter to infinity? Will the flash strength peter out after only a few meters? Is there a real zoom or that useless "digital zoom"? If I'm just shooting one quick picture -- like if I'm just using someone else's camera to get a shot of them with a friend -- I generally don't need to know more than where the shutter / focus button is.
Likewise, if I sit down at a friend's Linux computer to check Gmail, odds are I won't have too much trouble finding where the Web browser is. But the more work I want to do with an unfamiliar distribution, the more work I have to do, period. That said, it's their Linux install, not mine, so I gotta deal.
Similarly, I don't hear a lot of complaints about how digital cameras are markedly unalike, because we expect that from cameras. They're different by design. Also, we tend to pick one camera at a time and stick with it. It's ours, and we chose that one and got to know it because it had the features we wanted. If someone else is using it, it's up to them to figure it out.
The reason I'm drawing this analogy is to address a question that other people have brought up from time to time. Would the best thing for Linux be a uniformity of presentation -- not just in the GUI, but everywhere else as well, including things like package management? It's tempting, but difficult to enforce (to say the least). The other question is: What, and who, is that uniformity for? If it's for broader adoption of Linux, that's one thing. But if it's to "correct" something that may not even really be broken in the first place, that's another.
Another counter-argument is that while it might speed adoption of Linux, whether among nontechnical users or relatively sophisticated ones, it may end up harming the very thing that has made Linux so remarkable: its diversity. Obviously it's impossible to make all incarnations of Linux perfectly uniform. But maybe it's also foolish to expect that they should be.
(For another viewpoint, see Too Many Linux Distros Makes For Open-Source Mess.)