Best-guess answers, in order:
1) You can type 'rm -rf directoryname' to delete the directories.
2) You might have deleted something that X needed to run.
3) Deleting /tmp files is pretty much pointless. /tmp files are temporary. They're deleted (generally) whenever you shut down.
4) Again, you might have removed something important.
In general, if you start removing things without having a pretty good idea what you're removing, you're asking for trouble. The best way to remove things is to uninstall them ('rpm -U programname', in this case).
Having a "full" hard drive isn't always a bad thing. In fact, it's a sign that you've estimated your needs properly, and are getting your money's worth. On the other hand, you might have installed things you'll never use, too. As for your graphical login problem, it's not that you have a lack of space, it's more likely to be that you didn't configure it correctly. Maybe it's as simple as you didn't tell X to start up in runlevel 5 on boot, or maybe you picked the wrong card/monitor combination, or perhaps you loaded the 3D drivers incorrectly (NVidea drivers sometimes do this.)
If you want more space, maybe you can allocate more from another partition. I hate to suggest this, but for a newbie, sometimes it's easier to go through the install again to fix problems than to try and fix them manually after the install. Once you have a few weeks experience with a working Linux install, you'll be able to correct these kinds of problems with ease, but it may be too frustrating to have to do it that way now, and it may give you the mistaken impression that Linux is "hard to use". (It's not, it's just different, and that difference takes time to adjust to.)