Mac OS X
I've played with Mac OS X Develeper's Preview 4 for a few hours now, and have generated a few thoughts on the matter. Overall, the OS is a radical departure from the old MacOS, but I'm not so sure that the Mac community will embrace it with open arms. Let me preface all I'm about to say with the statement that I do realize that this is not even the "Public Beta" version of Mac OS X, and that things will almost certainly change by the time the final OS is released.
The "Aqua" interface is very flashy, and it's not as slow as I thought it would be initially. I did a clean install on a G4 400 with 128MB of RAM. The Dock is somewhat confusing to me, okay, make that very confusing. Sometimes there is a background behind the icons, sometimes there isn't. It seems to come and go as it pleases, I don't understand that. I did finally catch on to how to tell if an application is open, at least. There is an Internet Explorer icon on the dock, clicking it launches IE. Say you close your IE window, but not the application, you don't readily notice this, because that icon is always in the dock, regardless of whether or not the app is open. My boss pointed out that three periods are left under the app icon if it's running, which is somewhat subtle.
The icons are huge! I felt at times like I was using an operating system for idiot savants, with big shiny icons and illogical layouts. The control panel is a great example of this, all of the icons are along the top, and after you select one, the bottom pane of the window turns into the settings area for that control panel. All of them are a different size, so the window resizes as you flick from option to option, which is somewhat annoying. What's really annoying is that you scroll horizontally for about three screens to see all of the icons, I was unable to find a way to view them all at once.
The GUI felt responsive, but the apps didn't. I know, that sounds really bizarre, but I assume it's due to a lot of debugging code still being in place. Linux/BSD users will feel very at home in this OS, it's nice to be able to drop to a command line to view your processes and kill them at will, no more "force quit" after which your system is hosed and has to be restarted. I've been unable to access AppleShare/Talk servers, maybe I'm dumb, but it's certainly not even remotely intuitive at this point.
Despite all of these gripes, the OS still shows a lot of promise, but I find myself wondering how the hard-core Mac community will adjust to the changes. Apple claims that the user will never have to use the command line interface, but what about the support staff? :) The single thing I was most impressed with was the boot time, less than 10 seconds on the G4 I was using! Kiss those extensions goodbye!