The Debian has 2 (!) gigabytes of data. This is quite a lot for what is currently capable of, so believe there are tons of images or some sort of stuff (just don't tell me these are 2 GB of compiled code).
Yes, 95%+ of that is just executables ("compiled code"). 2 GB is in fact significantly below the average for a PC OS with GUI -- half the size of mainstream linux distros, which are still 1/2 to 1/4 the size of major commercial OS's.
Turn the GUI of if it's running on startup, and do not start it automatically
The last raspbian image I checked it's not on automatically, which is a bit different from normal debian, I think. It's enabled in runlevels 3-5, but the default is 2.
Uninstall useless stuff related to the GUI (eg. browser and so on, but not the GUI itself, in case I ever needed it)
You won't save much space that way, a few MB here and there.
But: glancing through output from du -h on the raspbian 2013-12-20 image, I notice a whopping 422 MB in /opt/WolframEngine. Google that yourself to find out what it is; in any case, it's not necessary. I don't know when or why raspbian added this to the base image (I doubt very much it is in the normal debian, or any other normal distro base...go figure).
Since apt-cache search wolfram only names one package, that's probably what you want to pull. Try:
apt-get remove wolfram-engine
Then check to make sure the directory in /opt is gone. That should free up enough space until you get a 4+ GB card. apt-get install wolfram-engine on raspbian sans the package wants "588 MB of additional disk space", part of which is oracle java 7. Meaning you likely also have oracle 7 installed and probably don't need that either (if you want an oracle java, use the 8 version which is compiled for the hard float pi).
