Having used all of the options already mentioned here for the translation of several different open source projects, I have to say that Transifex is in my opinion at the very top of my list. From the perspective of the administrator who is setting up a project for localization for the first time, the fact that you do not have to create several accounts for the given versioning control system is a major plus, specially for larger projects with many different teams and collaborators. Most projects will create these accounts/credentials on a as needed basis and try to keep them to a minimum. During "peak" season, usually right before a release, having only one (or a couple) person with credentials to make the commits could lead to a bottle neck if this person is not available for whatever reason. Transifex allows for the creation of a list of "trusted" collaborators who have direct access to the commit process without the need of a corresponding account in the version control system.

About your first point, I actually think that the fact that you only need a text editor, regardless of what operating system you're using is a major plus. No tools to download but the po file itself. I wasn't sure about your second point, but about the third, you could use roles to control the workflow, so that people can upload their work and then be reviewed by someone.

I don't remember right off the top of my head if Tx provides a diff of the uploaded file compared to the original one, but the Tx guys are very quick at turning feature requests into functional code.