OK so i admit I'm a lazy ass linux user. I never had that love for the console since the Red Hat 7.2 days.
So here's my short top 10 things that you can easily tweak in Ubuntu that you can do very easily using the GUI interface tools.
I'm specifying Ubuntu because in other Linux distribution the Gnome menu layout could be slightly different.

1. Make your fonts looks spiffier.

Usually if you have an LCD display the font config isn't set correctly and you have slightly edgier fonts. You can easily fix it by going to System > Preferences > Appearance > Click on the Fonts tab > Select Subpixel Smoothing. It should update in real time. Also it will crash/close your firefox session so you have to restart it.

2. How to share your home folder / any other folder easily. (Tested in ubuntu 8.04, 8.10)

Right click on a folder. Click Share, Select the name you want for your shared folder, select if you want or not writing permissions or if someone can access that folder directly without any password (guest functionality) . For the first time only it will ask you approve to install samba. The problem is that it doesn't tell you that you have to login/logout to be able to share folders after that (i really don't know if this is a bug),

3. Configure any boot parameters

So if you don't want to see a bunch of choices in your grub editor, want to change the timeout, the colors, prompt for a grub password etc, you can easily install Startup Manager

Click on Applications > Add/Remove > Select Show All available Applications (if you didn't already), confirm that choice > type in the search bar startup manager > check the package with the same name > apply changes.

Now the application will be in the System > Administration > StartUp-Manager.

See the application homepage for more details and some screenshots.

4. Enable Ubuntu Backports

Some packages remain fairly old in Ubuntu after a release is made, but thanks to the community the most popular packages get updated once a new release is made.
To enable this is fairly easy. Go to System > Administration > Software Sources > click on the Updates tab > Check Unsupported Updates.

5. Add a nice clipboard manager

Sometimes when you close a program from where you copied some text the paste becomes unavailble. To easily fix this and also have an history of past copies you can install the lightweight Parcellite clipboard manager.
Go to Applicatiosn > Add/remove > select all available applications (if you didn't do that already) > ype parcellite in the search box > Select the package and apply changes.

The nasty part about this is that you have to manually start it each time you start the computer. You can fix this very easy by going to System > Preferences > Sessions > Startup Programs tab > Click add > Enter Parcellite as the name of the program (so you know what program starts), Click browse for the command > Select Filesystem on the left side > double click the usr folder > double click the bin folder > select an entry and start typing the programs name, the search feature should take you to the parcellite entry > Click Open. On the comments entry you could add Clipboard manager (just to respect the gnome HIG :P) > click Add and that's it.

Remember that you can change parcellite's options by right clicking on it's icon from the tray and selecting Preferences.

Update: As somebody pointed up in the comments parcellite isn't in the 8.04 repository. You can search for glipper. It's very similar and the instructions match. Just replace parcellite with glipper :)

6. Automatically open the currently started applications when you start the computer.

Go to System > Preferences > Sessions > Options tab > Click Remember currently running application
If you always want to remember the applications that you have started you can check the Automatically remember running applications when logging out option.

7. To see the weather in your current location you need to configure the clock applet from the panel
Right click the clock, select Preferences go to the Location tab and add yours. Go to General tab and check Show Weather and Show Temperature. the Configure the weather tab if you want to change different settings like temperature options or wind speed options.

This wanted to be a top 10 list but meh. I can't remember what other options an average user needs.