Ubuntu release cycles are shorter and changes are subtle. Over the last 6 years there has been 11 Ubuntu releases, once every six months. When you compare that to commercial, proprietary Operating systems, Windows or Mac OS, release cycle of ~2yrs its quite an impressive feat. As impressive as a six month release cycle might be it would be disingenuous to compare Ubuntu release to its commercial counterpart. Ubuntu, just like all other Linux distribution, relies on work done by developers outside Ubuntu ecosystem (ie, Linux kernel, GNOME) to churn out frequent releases.
The advantage of short release cycle is that you are always up to date with the most secure and best version of the software; but this also has the disadvantage of not having enough time to do extensive tests to minimize issues with the final release. Even with Feature Freeze and stages of alpha and beta releases stability issues creeps in to the final release. So, its a good idea to wait couple of weeks before upgrading to the latest release.
Ubuntu has come a long way since its first version. Development process has been much more streamlined and organized. Over the last few releases the developers are concentrating more on speed and User Interface, as a result, with Lucid Lynx we will see some of the biggest changes to Ubuntu to date.
Lets take a quick looks at how Ubuntu has transformed over the years: