I've been using KDE for years. I can't remember exactly how long, but I vaguely recall getting excited about a magazine which had KDE2 on it's cover disc, so I must have been using it since the later versions of 1.

Some stuff that I like about KDE4 are (in no particular order):

o Dolphin - it's a fantastic file manager which has all the features you'd expect from a powerful modern file manager yet in a clean and uncluttered interface. Yet everything is easy locate and enable as and when I need

o Unified / Integrated components - I can be running Dolphin and hit [F4] and a Konsole terminal pane will attach to the bottom of the file manager; already cd'ed to the directory I'm browsing. And as it's KDE Konsole, all of my profiles et al from the standalone terminal emulator will be available.

o Taskbar - while I might dislike Windows as an OS (personal preference; not trying to start a flamewar), I do think the taskbar was a stroke of genius. The Windows taskbar peaked at Windows 2000 / XP (minus the themes). The KDE taskbar has the same clean layout that I liked from Windows 2000.

o KWin Compositing - I used to dislike KWin and much prefer Compiz, and in many ways Compiz is still probably preferable. But these days KWin is actually pretty decent on it's own; which is handy because I don't have a whole lot of time to spend making a computer look pretty. So it's nice to have an "out of the box" solution that comes with KDE4 rather than having to install lots of additional glitter manually.

o Krunner - [F2] then type any application or document name and it pops up. Hugely convenient.

o Configuration - I work as a Linux and UNIX systems administrator by trade, so sometimes I just can't be bothered to configure things the "proper" way if I just need to quickly add a printer to CUPS (for example). KDE comes with some pretty decent control panel applets for desktop administration. Though I really dislike how they've grouped the items in System Settings - which makes it just as counter intuitive to find items as it is in Windows Control Panel when you're unfamiliar with Windows.

o Snap Regions - I know most WMs support that these days, but it was less common when that feature was released in KDE4.0 (or at least that's what I seem to recall. I might be wrong about that).

o Sane defaults - for the most part, I find I don't have to do much to make KDE usable.

I know most of what I've described can be done in other WM/DEs, but I've always just felt at home in KDE. Maybe part of it is just because I've been using it for so many years on so many different distros (SuSE, Arch, Mandrake, Slackware, etc) or maybe it's because many of the distros I've ran on my desktop have been KDE-centric (eg SuSE, Slackware) or at last been one of the few to do decent packages for KDE (I'm still not convinced KDE4 on Kubuntu isn't total garbage where as Arch seems to suit KDE well). But KDE just feels like home.

The only other environment I've felt comfortable in was Awesome, which, weirdly, is the polar opposite of KDE. Most of the other DE's look pretty but I'm quickly bored of them (which I'm guessing is exactly how you and many others feel about KDE4?)

As for if I discovered Linux today; who knows what I'd be using. Probably Awesome because I'd want to think of myself as "l33t".

Good luck with your DE/WM project by the way. That's quite a daunting project you've taken on - but I can imagine it'll be great fun too.