Interestingly IEX's application has attracted 388 comment letters, with 99% supportive and Citadel's one of the handful against: http://www.nanex.net/aqck2/4709.html
For contrast, the BATS application received 3 comments only.
You could argue that these are direct competitors of IEX and therefore have a vested interest in preventing the new exchange - or you could argue they are best suited to see the holes in the application.
Keep in mind that previous exchange applications have went through without this level of discussion. You could argue that this is because the other exchanges are "scared" of IEX. Or... consider the possibility that IEX's application has some faults. For example, much of the focus has been on IEX's smart order router not following the same rules as all the other exchange participants. The argument is too nuanced to summarize here... but you should never decide an argument based on number of responses!
BATS when it became an exchange was nothing but a vanilla competitor marketed as a Better Automated Trading System. It didn't have anything like a speed bump.