XFCE is bloatware. It provides precious little performance improvement compared to GNOME (at least in my experience), but with the loss of considerable functionality and a hundred mildly obnoxious quirks that combine to make it not at all worth my while.
On the other hand, I quite like LXDE. It starts instantly, it is blazing fast and is very highly configurable. It also has a clean, uncluttered interface that gets out of your way. PCManFM (the original, and central, component of LXDE) is hands down the best file manager ever written, lxpanel is both lightweight and very functional, and of course there is the power and flexibility of OpenBox underneath. What's not to like?
I agree with gamefreak's assessment- right now LXDE is the best lightweight desktop environment available. If XFCE continues on its current trend, it will no longer be considered a lightweight desktop environment at all, and will simply become a third heavy desktop alongside GNOME and KDE. Of course, LXDE has been growing in size as well. Hopefully they'll stave off the feature creep for many years to come.
Friskeygirl- LXNM is not yet mature or functional. I use GNOME's network manager in LXDE. You have to set it as a startup application unless you want to manually start it every time you log in. Another popular choice is to use wicd for wireless management.
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