I take it you have a new Mac. Don't bother reinstalling the OS until you have to ... it's just a waste of your time. Just delete what you don't want (it'll leave a few preferences files lying around, but to be honest, you won't notice the few MB they'll take up).
Apps themselves don't take up that much room. But GarageBand and iDVD support files are the biggest disk space users. You free up most by removing GarageBand and it's support files in /Library/Application Support/GarageBand and /Library/Audio/Apple Loops/Apple/Apple Loops for GarageBand.
You can remove the MS Office trial with its uninstaller.
You can remove the iWork Trial by deleting it.
You can delete printer drivers you won't use ... /Library/Printers
The largest remaining "waste of space" is multiple languages. Use Monolingual to remove unused languages - do
not use it to strip out PPC architecture files.
I bought a Mac Mini recently that I use just as a media server. I removed all the iLife apps, GarageBand, printer drivers, trials etc, and it's using up 4.5GB. I could reduce it to about 3.5GB with a completely minimal install, but what's a media server without iTunes and Quicktime?
Are you sure about that 30gb figure? That sounds very high to me.
I'm guessing it's the perennial "I'm supposed to have a 160GB drive but there's only 130GB free, on a new Mac!" comment.
To the OP, usually, out of the box, 15GB is used up. The rest of the "missing" space is the difference between GB and GiB.