I think the main difference is that those sites are geared towards outputting a ranking or score of some time. Takehome is geared at giving you a way to qualitatively assess the skills of each candidate and give you something meaningful to discuss with them in a follow-up interview.
Also, all of the existing sites I've seen have one or more of the following problems:
* Make you use their editor
* Make you install their software on your computer
* Make you use some specific language
* Make you use their set of challenges
Takehome uses nothing but git. Whatever you put in the repository and how you get it there is up to you.
That holds true both for the candidate and the challenger. Takehome encourages challengers to create their own task matching the kind of work/language/framework/etc that's relevant to their team.