The operating system's new, mobile-inspired Unity interface, in particular, has displeased a significant proportion of longtime users for its divergence from the software's traditional GNOME history.
Those who don't like Unity have myriad choices, however. The most dramatic choice, certainly, would be to switch to another Linux distribution. In that case, I'd suggest Linux Mint as an extremely worthy contender--it's based on Ubuntu, but hasn't adopted Unity, to the relief of many.
It's not actually necessary to switch away from Ubuntu to get a different desktop, however. If you're happy with everything in Ubuntu but Unity, it could be well worth your while to try one of these alternatives.
1. Classic GNOME
One very smart move Canonical made in creating Natty Narwhal was to build in the option of returning to the classic GNOME desktop. That option will reportedly disappear in Ubuntu 11.10, or Oneiric Ocelot, but for the time being users unhappy with Unity can go back to this good, old familiar GNOME.
Users who have enabled automatic login will need to first log out of their current Unity session. Then, at the login screen, they need only click on their user name and select the "Ubuntu Classic" option from the Sessions drop-down box.
Those who haven't enabled automatic login can simply click on their user name at login and choose Ubuntu Classic.
KDE is arguably the best-known alternative to the GNOME desktop, and it's what you're already used to if you run Kubuntu instead of the standard desktop Ubuntu.
KDE and GNOME differ primarily in the way things are presented on the desktop, including different color schemes. GNOME's desktop has a reputation for being simpler, as I've noted before; KDE's approach has also been described by some as more Windows-like.
To get KDE as a login option on your machine, go to Terminal mode and type: sudo apt-get install kubuntu-desktop
Xfce is a lightweight desktop that's used by default in Xubuntu, the Ubuntu variant designed for low-specification computers. It focuses on using minimal resources while offering very quick speeds. To get Xfce as an option on your Ubuntu computer, you need only go to Terminal and type: sudo apt-get install xubuntu-desktop
4. Unity 2D
Finally, it's worth noting that if it's mostly speed that's bothering you about the full, 3D Unity desktop, the 2D Unity option is generally quite a bit faster. Unity 2D doesn't come bundled with Natty Narwhal, but directions for installing it are provided on the Unity 2D wiki page on the Ubuntu site. Alternatively, you can also go to Terminal and type: sudo apt-get install unity-2d
One of the best parts of Linux is the way it lets users make their operating system their own. In this regard--as in most others--Natty Narwhal is no exception.