For many years, Xubuntu's desktop Xfce has billed itself as a lightweight alternative to GNOME and KDE. However, with the addition of Lubuntu to the official variants, Xubuntu is perhaps best considered as a compromise between Kubuntu's complete desktop environment and Lubuntu's speed.
Recently, with many people discontented with GNOME 3 and Unity, Xfce in general has found a new popularity, especially with Linux founder Linus Torvalds endorsing it as preferable to the GNOME 3 series.
Xubuntu is one of the most prominent distributions featuring Xfce, so it is well-positioned to take advantage of this new found popularity. Its software list is similar to Lubuntu's, often favoring light applications, or ones with basic features, although it uses Firefox rather than Chromium for a default web browser.
The main advantage of using an official variant is that each is available from your Ubuntu software center. The only possible confusion is that each package is named for the variant, rather than the desktop it uses, so you have to search for lubuntu-desktop rather than lxde-desktop, or xubuntu-desktop rather than xfce-desktop.
However, once you have installed a variant, it is added to the list of available interfaces when you log in. Although the logo displayed during bootup will be that of the last variant installed, you can easily make comparisons, logging out and switching interfaces in a matter of moments.
Nor are the variants the only choice. Ubuntu 11.10 also includes gnome-shell (GNOME 3.x) and gnome-session-fallback (GNOME 2) for those who want to remain within GNOME. A long list of other interfaces, from IceWM to Ratpoison, are also available within the repository.
The only trouble is, such alternatives tend to have smaller groups of developers than the variants. At times, they may be less polished, and few of them have a separate mailing list - as the variants do – where any requests for help are likely to get a quick reply.
In general, if you're searching for alternatives within Ubuntu, then the variants are the place to start. More than any other alternatives, they give you the option of remaining within Ubuntu while finding the exact options -- and sub-community -- that you prefer.