Is there anything that Ruby on Rails Developer can't do because of the MIT license? Can't he/she buid web applications to sell for people for example like in PHP?
The MIT license is one of the most permissive licenses, which is good news. Essentially the requirements levied on you are to:
It also has the standard clause in just about every software license that states there is no warranty of any kind.
All permissions are granted free of charge, and you can copy, extend, distribute, sub-license, sell, etc. your code that is based on this MIT licensed project. You can keep your project closed source and there will be no restrictions requiring things to be otherwise.