Whilst Ashley's answer covers the requirements, I think it's worth knowing why this is the case in the first place. Here is my understanding of the reasoning behind the reputation barrier:
The 'minimum rep' idea steams from Stack Exchange's paradigm of: "Ask questions, get answers, no chit-chat". The idea of Stack Overflow (and later, the entire Stack Exchange Network) was to bring the useful information to the forefront, without all the surrounding noise:
...There's far too much great programming information trapped in forums, buried in online help, or hidden away in books that nobody buys any more...
- Coding Horror: Introducing Stackoverflow.com (Jeff Atwood)
This is still the case to this day, to the point where it's ingrained in every site's Tour Page:
This site is all about getting answers. It's not a discussion forum. There's no chit-chat.
As such, comments aren't treated the same as questions and answers. They have no edit history, if they are removed they are deleted permanently (as opposed to soft-deletion). They don't 'bump' questions and aren't handled by the review process, and thus can go unnoticed for a while.
Comments are second class citizens on the Stack Exchange network, not designed to hold information for all eternity. They may get cleaned up at any time. Generally, truly important information should be incorporated into an answer of its own anyway.
- Meta.SE: Why do I need 50 rep to comment?
Judging from the majority of answers we currently get from low-rep users, most comments would say something along the lines of "I have the same problem", "I agree" or "Thanks, this helps". These do not add any value as either answers or comments, and would have to be manually processed as comments. It would also be a problem with spam accounts if they were allowed to comment from the get-go.
Thus, the 50 rep limit is to help reinforce the question/answer paradigm and to reduce noise.