Linux and Ubuntu has come quite a ways in the past few years. There are millions of users who have switched to it and are using it for their computer needs. While it is a great operating system, there are a few things that some complain about which are things that many wish worked with Ubuntu properly.
The first issue that many have which is at the top of the list for most Linux distributions is that full screen Flash is not working properly. When a video is full screen in a browser that is a Flash file, there are flickering issues. This has been a long standing issue with Linux that is not yet fixed. If one downloads the Flash file, it works fine full screen, but many would rather it work in the browser for popular video sites.
Certain hardware does not work properly with Ubuntu. The reason is because there are manufacturers who do not have open source drivers to let their hardware work. This is more an issue with those who make the hardware than those with anyone on the Ubuntu side of things. If the printer company or video card company refuses to work with those on Linux systems, they are the ones not allowing it to work properly. Find an alternate way of making it work or switch to hardware that already works.
At times there are complaints about certain software not running properly. Most of the time this is Windows software that does not run in the Ubuntu environment. This is not Ubuntu's fault at all since it runs all Linux software fine. The solution is to move away from Windows software primarily if you are wanting to use a Linux system. Plus virtual machines work great and will run Windows software if that's what you need to do.
While there are these changes that many would hope for, there are also many practical solutions for what may be missing.