Debian was my first and only distro until Ubuntu hit approximately 6.04. It was then that I felt Ubuntu offered a smooth enough experience to justify the "bloat" (funny how perspectives change) that came with the switch.
Having used mostly Ubuntu on my servers for the past seven years, there has been the odd time that I needed to squeeze Linux onto a small flash, and it was Debian to the rescue. Debian is great, but when you're used to Ubuntu it does feel unfamiliar in the way it handles some things.
A second thing that has me still preferring Ubuntu on servers is the quicker uptake of new features. SSD TRIM is a big one, as I started migrating all of my systems to SSD in 2008, and TRIM required the newest kernels. Yeah, I could have compiled my own kernels in Debian, but as any Debian user knows, updates are a different world when you step outside the Garden of Eden that is apt-get. Ubuntu made getting such new features a piece of cake and in a timely manner.
And lastly, there are advantages to being mainstream. There are tonnes of cool products being developed for Linux these days, and generally speaking, Ubuntu and RedHat/CentOS are the first distros to get support. Steam is one example. LTSP is another project that you're going to get way better developer support on if you're running Ubuntu. A counter example is VoIP software, including FreeSwitch and a bazillion Asterisk distros, which tend to be much better documented on CentOS.