Well - YDL 6.0 is quite a change from previous versions of YDL. Specifically, the codebase has shifted from Fedora to RedHat Enterprise Linux (via CentOS). So - it's not quite as polished as YDL 5.0.x.
On the other hand, it should pay good dividends in the long run - CentOS is very stable and code is upgraded on a much more conservative schedule. One of the downsides is that it doesn't include as many packages as Fedora - it's designed for maximum stability. That's probably why you can't load Yumex by default.
So - that's the "why" of what some of the differences are - now let's look at your questions. You mention Ubuntu and the ease of adding software to that distro. That is because Ubuntu has a slightly different concept of copyright law - they make deals with copyright holders, etc. - that allow them to include software like MP3 in the distro.
All RedHat based distros - Fedora, CentOS, YDL, etc - only include fully Open Source code in their distros. Specifically everything in YDL uses the GPL license.
I discuss all of this stuff ad-nauseum here:
http://www.yellowdog-board.com/viewtopic.php?p=11293#11293
http://www.yellowdog-board.com/viewtopic.php?p=10239#10239
http://www.yellowdog-board.com/viewtopic.php?p=13884#13884
http://www.yellowdog-board.com/viewtopic.php?p=9996#9996
So - the RedHat solution is to have users add manually repos that include these extra programs. The other replies to your question describe how to add them.
One thing about YDL is that they use the E17 (aka Enlightenment) window manager by default. You can change this at the login screen using the Session command. You should try Gnome - that's what Ubuntu uses. This will make the system seem a lot more snappy. If you need it snappier, try using Xfce - it's available for YDL 6.0 from the extra repos.
So - in summary:
1. Add the extra repos to your YDL 6.0 installation.
2. Switch to Gnome from the login screen.
3. (optional) add Xfce and try that.
Finally - it's very easy to find 5.0 - it's on all the major YDL mirrors. And these aren't torrents either. If you do decide to downgrade to 5.0 - I'd suggest 5.0.1. It seems to work the best on the PS3. You will still need to add the extra repos to 5.0.1 to unlock all of the functionality.
Mirrors here:
http://www.terrasoftsolutions.com/support/downloads/
Cheers,
Paul