Launchpad equivalent for Debian
Your question made me think of this letter by Debian's founder Ian Murdock :)
In theory Debian will use the exact same package management system that Ubuntu does. If the words apt-get or Synaptic sound familiar, then you already know how to use Debian's packages. As a matter of fact, most of the packaging is done by Debian's developers, and Ubuntu will clone them (often unmodified) in their repository.
Now I'm not sure, maybe you only use Ubuntu's "Software Center" to install your apps. As far as I know, Debian doesn't have such a thing. But again, all you need to know is to install packages with apt-get and you're good to go.
Note that I've stopped using Ubuntu since 10.04, things might've changed a bit.
Very few distros today will actually require you to compile your packages yourself. The ones that do will clearly state so, you won't get any bad surprise.
And before you do anything, try Debian in a Virtual machine. You'll know exactly what to expect.
TL;DR: Debian is Ubuntu's direct father. It uses the same package management system. You'll feel perfectly at home.
Apparently Debian does not have something like Launchpad PPA, since latest packages can damage system's stability. Instead it has a separated approach (assuming one is running the stable release and in the order of decreasing stability):
Backports. Here one can find somewhat newer but not the latest version of some packages without compromising the system's stability.
Testing. Once can convert to testing and then will access to quite the newest versions of packages. Fairly compromising system's stability.
Unstable. Once converted to, the bleeding edge packages are here. Greatly (at least that's what Debian says) compromising system's stability.
3½. One can also mix repos from all of the above and have a mixed release, however (as far as I know) this is strongly discouraged.
I knew that Debian had this approach but what I had thought was that I could use stable and use some packages from the unstable repos, thus having a mixed release. This is not the way to go in Debian. Currently I am using Wheezy and actually very satisfied with it. No issues so far.
Curiosity: So, if you think about Ubuntu's repo approach, it is totally wrong considering Debian's! And one should consider Debian's approach as Ubuntu is a Debian based distro. However, I used Ubuntu 10.10 for more than 1½ year (with a lot of packages from Launchpad) and it didn't really crash on me. But that's just me and one specific distro release.