More specifically, it's difficulty of learning a language, as Wikipedia correctly calls it. This topic always generates a lot of discussion, mostly personal opinions, which I won't deal with. To streamline this discussion, I only focus on metrics that are measurable and on aspects most people usually neglect.
Difficulty is not an objective measurement. It must be subjective and human nature contributes to its measurement. So what is possible to measure something subjective? The answer is survey and statistics, as done in social sciences in the last century, and some humanities in recent decades. A wide spread measurement on the Internet is the student study time statistics from Foreign Service Institute (FSI) of the US Department of State, available at Wikipedia, Effective Language Learning, another forum, and one of the earliest, in here, but not at the FSI web site.[note]
[note] In April 2010, I contacted Defense Language Institute (which assigned the actual language training to FSI) for the official web page showing the student study time broken down to individual languages. The gentleman sent me back a PDF file of that same information, but told me that was not available on their web site.