Hmm, well first of all, I would like to say that I think that it's very nice and simple signature, except I just have a few things to say. :x
Firstly, you notice how the render needs a lighting because the right side of the Joker's face is lighter, while the left side is darker, which means that the light source is coming from the right side. So I think you could have used the render wisely and added a light source.
Secondly, I like the background and everything but it only blends in with the render a little bit. Was the background meant to go everywhere or was it meant to be like surrounding the Joker, giving it more emphasis? Because to me, it just looks like you did the background first then just put on a render. ( No rudeness / hatred intended / offense intended. ) But if that's the case, then it's fine the way it is. Although, the effects in the background are very nice indeed, I would've just liked to see some of the effects cover the render a little.
Lastly, like everyone said, the text. It's not that visible. ( No, I'm not being a racist. ) The font goes absolutely well with the render, and maybe could have been used as a pattern for the tag to repeat it self, giving it more of a emphasis.
It's a nice tag overall, but I've seen better from you Zack.