Well seeming i got asked nicely by slash to comment on this i think i better.
It really isnt that bad for someone who has just started making graphics to be honest. alot better then my sigs when i first started getting into graphics.
1. Try and not to over brush on your backgrounds , grungy backgrounds are all good and well but when you over brush on them.
2. Text could do with some work . but i am not very good with text still so i cant really say anything but try and keep away from beveling your text if you can.
Tip : Always try and make sure you have a focal point in your graphic/sig what draws peoples eyes to it . i have noticed in your sig you have lighting ( white brushes behind mario ) what is all good and well but it takes away from the white hand of mario if you get me if you just put it behind his body that would have looked better. ( but i aploud you for trying lighting on your graphic )
Anyways hope this review and tips help a little , if you ever need any help with anything then just ask and i will try and help . that goes for anyone on here really.
Also one little thing dont be afraid to use tutorials , alot of people out there when starting to make graphics bypass them for some reason but they can really help when starting out making graphics.
( sorry didnt really mean to type that much out but i got carried away )