I was gonna go through each sig and specifically critique but yours is really the same as Vegeta_Rules' where all the sigs are missing the same things.
So with flow, there is none. Now that's not necessarily some terrible thing that you need to go cry about or something. If you have something with absolutely no flow that still looks good, then go for it. You're the artist, so do what you want--screw the people.
But with what flow actually is...example time.

See how all of those explosion lines are coming out of one place and flowing outwards? That's an example of flow. And also notice that your eyes go to the render before they see the text (which is hard to see anyways imo). That's the ideal flow for a sig that size. Render --> Text --> Extras.
Depth...you're missing it toooooo. xD
No shadows...no nothing. It's all on one dimension. It looks like a sheet of paper. Again, not necessarily a bad thing. But depth can help so much.

People have told me that this sig exemplifies depth very well. You can tell that the explosion is in front of Knuckles because of shadowing, and you can tell that he's also pretty far in front of the wall behind it, because of the shadow I put behind it. This [depth] is pretty hard to do with just tips. Experiment.
And then the rule of thirds...
You haven't really done anything against it, but then again you've done nothing for it.
Think of your sig as a grid. You can see how there are two vertical lines within it. Place your render on those lines. This helps with flow as well. The horizontal lines don't matter as much, so don't worry about them.
_______________________
|______|________|______|
|______|________|______|
|______|________|______|
So yeah, if you have any questions feel free to ask.