I like the fact that Ubuntu has variants - despite being such a popular distro it has space for Xubuntu, Lubuntu, etc, most of which fill different niches to the main Ubuntu. It also makes sense for the project - if people stop using Ubuntu for some reason (*cough* Unity...) there's a chance they will still use a *buntu variant.
On the other hand, I've switched to Linux Mint and really like it - enough of an Ubuntu base for a good package base, but more stable and better for "Aunty Tilly" - you can have a very simple start menu showing just the few applications someone needs to use most of the time.