You don't tell us what program you are using. Firefox 3 has some problems on
some people's systems. Have you tried a different browser such as Opera or
Flock? If it is a media player such as Rhythmbox or Mplayer, try a different one
such as VLC or Amarok. It could be a bug in one program or something about your
configuration. In other words media plays fine on Ubuntu 8.04 and 8.10 as I have
both installed and can play anything except ape files (monekey audio).
As far as the difference between Kubuntu and Ubuntu, they are different
environments using different programming languages, use a different display
manager and use different applications. You are comparing apples with oranges.
Some people prefer mixing the environments by running KDE apps in Gnome or
running Gnome apps in KDE. Truth be told, there are far fewer Gnome apps worth
running in KDE than the other way around. K3b bests Brasero, Amarok bests
Rhythmbox, Konqueror bests Nautilus, and now that Gnome has dumped Pidgin in
favor of its own IM, Kopete is bound to gain favour. These are only my opinion,
but I can't think of (m)any that go the other way. Of course, it is all about
personal preference. Many Ubuntu users prefer simplicity and find KDE as being
too busy with too much choice and configuration. Many KDE users find Gnome to be
frustrating and limiting. It is all about what you are used to and the way you
want to work.
My own opinion is that I use both. I am in KDE now, but switch to Gnome for a
change. Since I tailor each to my way, I sometimes need to do a double take
before I see which one I am in, as they resemble each other. I think that out of
the box Gnome is plain and quite ugly, but it can be dressed up and made
elegant. KDE is more attractive, has more features and has better applications,
out of the box. That is just my opinion; others will tell you differently.