I've pretty much always used KDE since it's the default during Slackware's install.
I use the default configured applications, mostly like franckbell and enorbet said. Those I use daily include:
- Dolphin, the "expand folders" ("details" view) and the preview on hover are pretty nifty features, I really miss them when I'm on Windows at work, and I've never liked Konqueror
- Konsole, I'm found on the command line and tabs are so useful
- Lancelot, I tried and adopted it really quick instead of the default launcher, I'm a huge fan of "activate without click"
- Kate has the usual bells and whistles of an advanced coding/text editor (I use Notepad++ on Windows)
- Gwenview, Okular, Amarok, Ktorrent just fit my needs
And some additions from SlackBuilds.org
- kaffeine, TV viewer (dvb)
- digikam, photos library and all those Kipi plugins
- tellico, books, CD, DVD collections
- kdenlive, video editing (not that daily)
- kde_cdemu, to mount ISO images without switching to root in Konsole to use loop devices
As for the non-KDE applications, I stick to Firefox for web, Thunderbird for mail, and VLC (AlienBob's) for videos.
I don't really use the "semantic desktop" (balloon / nepomuk) as I'm old school and tidy up my files with Dolphin. However the Lancelot's search box makes use of it, and I often use the "Alt+F2" shortcut: type anything, be it an application to launch or a file to search and here it is.
I don't use the "activities" either yet, though it has a certain appeal.
For the look'n'feel, KDE system settings allows to download, install and apply themes, icons, etc. from kde-look.org
directly: no need to download separately and fiddle with config files.
You can even make SlackBuilds to package for system wide installation
oxygen-gtk (part of ktown), kde-gtk-config (SBo) and "apply colors to non-KDE4 application" suit me, even with the light on dark theme (Zion reversed) I use.
One "beware" thing : KDE seems to ignore the X keyboard layout (/etc/X11/xorg.conf.d/90-keyboard-layout.conf), so I have to modify /etc/kde/kdm/Xsetup for the login dialog (KDM, runlevel 4) and configure keyboard layout in KDE system settings.
BTW I'm running KDE 4.13.1 on Slackware64 current, all the "desktop effects" work like a charm with my NVidia GT520 and its proprietary drivers.