(For those of you unfamiliar with what X11 is, it’s essentially the protocol Unix/Linux systems use to control their interfaces. For more information, as always, Wikipedia is your friend.)
As you may or may not know, Mac OS X includes an X11 server, no doubt in deference to its Unix roots. This is perceived by many to be a Good Thing™, as one can run all of one’s favourite graphical Unix applications on a Mac with the minimum of reprogramming required. The bevy of developers writing open source applications (free in whatever sense of the word you like) for the various Unices can produce Mac versions and we, the Mac users, can download and use them. The GIMP on your Mac: check. OpenOffice on your Mac: check. All through the beauty of the X11 server. You get all the fruits of open source software development right there on your Mac.
Unfortunately, you also get all the fruits of Unix open source interface designers right there on your Mac.
It is a well-documented phenomenon that interface design and usability in general of open-source software tends to be bad. Like really bad. Similarly unfortunate, albeit in a slightly different way, its conventions are very different to those of a Mac. On a Mac, you press ⌘+W to close a window: when running an X11 application, this tries to shut down the X11 server. While this does at least prompt you when you have applications using the server, it’s not overly pleasant.
So what’s my point with all of this? I suppose my point is that, while having an X11 server included with OS X is probably a good thing overall in that it allows me to get hold of decent (and free) image processing and office applications, it’s also something of a problem in that it makes it too easy for open source developers to produce lazy ports of their applications, leaving us with clunky apps which just don’t feel “Maccy” (both of the applications I’ve mentioned previously, plus many others). So my message to all the thousands of developers of high-profile Unix software (ahem) my message is this: if you’re going to produce an OS X port of your application, please take the effort to make it a good one, make it fit in with the OS X conventions and themes (look, Apple have produced reams and reams of documentation about this) and make it really feel like a Mac application. Do this and I will love you forever.