This is something I just noticed while reading through marc w's post over at USSM. Hopefully he doesn't mind me using one of his photos.
Look at Robles' back leg. Then look at his front leg. Then look at the catcher.
Ordinarily, these three points are supposed to arrange themselves in a ~line. Check out Roy Halladay as an example. This is what's called the driveline, and as much energy as possible should be going towards hurling the ball along it from the rubber to the plate.
Robles' right leg, though, doesn't swing all the way around to the line, instead stopping short at a...70? 80? degree angle. To make sure this wasn't a one-time fluke, I looked at some video available here and here, and, sure enough, it's consistent. Robles' right leg is very frequently - if not always - kind of off to the first base side there, turning the driveline into a triangle.
What does this mean? Understand right now that I do not possess any special knowledge of Mauricio Robles. What follows is simply speculation based on my own experiences as a pitcher. Having a front leg that lands too 'closed' can serve to inhibit forward momentum. The pitcher can end up kind of throwing over his leg, and he will have difficulty getting good forward extension. This can manifest in lower perceived velocity, difficulty in achieving a consistent release point, and difficulty in staying low in the zone. With the leg off to the side, there is no smooth delivery straight from the mound to the catcher. There's a delivery that's forced to compensate for a little resistance, and that can make it very, very hard to release the ball as far forward as you'd like.
There's also the risk of knee damage, as the knee is twisting and taking force and everything.
Is this what's holding Robles back? Is anything holding Robles back? I can't say whether or not such an observation matters in this particular instance. If nothing else, Robles is at least pointing his front foot in the right direction. But it's worth noting that Robles has exhibited some fly ball tendencies while battling inconsistent command. The signs are there that Robles' closed front leg is having some sort of effect.