Say you are tired of KDE in Kubuntu, you just "apt-get install gnome-desktop". You are tired of Apache, then "apt-get install lighttpd". Defaults are not limitations.
And seriously, most people pick one distribution and stick with it. I'm using Debian for everything ... work servers, laptop, home station. OK, to be honest the home station has Ubuntu on it, but that's still 95% Debian.
And, if there's a feature missing (like hardware limitations), you are most likely to fix it with a kernel upgrade, or with a patch someone on the mailing list gives you for free.
Linux distributions, and even versions of Ubuntu itself, are not organized in a "tier" layout, where Tier N has X more features than Tier N - 1, or Tier N's features have X more capacity.
Linux distros and Ubuntu versions, on the other hand, are organized horizontally: They don't have better features (per se), but different features.
Ubuntu
Kubuntu
Edubuntu
Xubuntu
Lubuntu
Fluxbuntu
Mythbuntu
Ubuntu JeOS
Ubuntu MID
Ubuntu Netbook Remix
Ubuntu Studio