It immediately runs a script from a site in a bash instance. The script could do anything like exploit some zero day like shellshock or other vulnerability. They would prefer the user read the script first. Most people won't read it either way, but if people don't just pipe it to bash other people will feel like it is more secure. If the script is served over http then there are also ways of replacing it by some mechanisms without you knowing as well which can add danger.
Running untrusted code without even looking at the code first. For all you know, the script asks for root access, and if granted, installs a rootkit, and if not, deletes all the files in your home directory.
Random scripts from the internet should always at least be casually reviewed. Posting something like this just encourages people to trust random scripts on the internet, which is going to end poorly eventually.