You actually can get OS X down to about 3 gigs without doing a clean reinstall. I've done it before, although it is easier on a ppc mac since you don't have to worry about accidently destroying rosetta.
You may want to look into a program called
appzapper. It's sort of the equivalent of the windows add/remove programs dialog box. Note that it does cost something.
The whole reason that windows requires that add/remove dialog box is because of the registry. With macs there is no registry and most apps are self contained so they can just be dragged to the trash. As other have said some apps have additional files, logs, folders, etc... found throughout the system folders. I've found that a spotlight search with the name of the app is generally enough to get rid of those.
Finally, that extra 18 GB of stuff isn't bloatware. Most of it is the iLife suite, which I certainly wouldn't qualify as bloatware.