Ubuntu 12.04 Update:
In the latest Ubuntu Precise Pangolin 12.04 LTS, the classic option is back. All you need to do is install gnome-panel like this.
sudo apt-get install gnome-panel
Logoff and choose classic from login screen. A Note: It is still believed to be in work in progress, not 100% ready yet I believe.
Ubuntu 11.10 Update :
If you are coming here to revert or remove gnome3 or unity from the latest Ubuntu 11.10, then the original solution in this page (found below) is not for you. To get a gnome classic like(not exactly gnome2) experience in Ubuntu 11.10, you would have to install the gnome fallback session as shown below. This infact is a fallback of the gnome3 shell.
sudo apt-get install gnome-session-fallback
Logoff and choose gnome classic from LDM. Also to add applets in Oneiric Ocelot, you would have to do a Alt + Right Click on the panel instead of simple Right Click as it was in Gnome2 (God how I miss it).
A little more detailed info to give gnome3 a ‘classic’ look and feel is provided in this post
For Ubuntu 11.04 users to recover from a broken desktop post installation of gnome3, the solution is given below.
If you installed gnome 3 from ppa:gnome3-team/gnome3, it disables or breaks all other desktop logins be it Ubuntu Classic or Ubuntu default (Unity as in case of Natty). For a sec, I panicked as I use a shared computer, and trust me gnome3 is not yet ready for a newbie user. Although, I was surprised to note that removing gnome3 did prove tricky. However thanks to this post It was a simple task.
Follow the below steps to remove gnome 3 and revert to Gnome 2.x.
sudo apt-get remove libgtk-3-common sudo apt-get install ppa-purge sudo ppa-purge ppa:gnome3-team/gnome3 sudo apt-get install gnome-panel
Reboot and you should be back to good old classic gnome. In case you don’t find gnome back again. try this.
sudo apt-get update sudo apt-get upgrade sudo apt-get install ubuntu-desktop sudo apt-get install gnome
I would try to lookup a post which allows us use gnome 2.x and 3.x together. If there is a choice I would always go for gnome3 > Unity.
Linus Torvalds hates Gnome 3, calls it a unholy mess!! http://www.theregister.co.uk/2011/08/05/linus_slams_gnome_three/
Also he had demanded that there be a fork for gnome2, and this one looks like one of them.
omgubuntu.co.uk (OMG! Ubuntu) has confirmed that development for Gnome2 fork ‘Mate’ is underway. http://www.omgubuntu.co.uk/2011/08/gnome-2-forked/
The Mate(gnome2 fork) is installable in Ubuntu 11.10/12.04, the instructions are given in this article here.
Oh and another alternative is to try LXDE
sudo apt-get install lxde