First, let me preface the following remarks by saying that these boxes look really good overall, especially for a first time comp. What I am about to say is just my opinion, and should be taken with the grain of salt it is intended.
Chicken,
If I were a table captain and saw that 6 in chicken appearance, I would have asked the guy if he were sure. I would then keep and eye on his scores for the rest of the categories. If they tended to be that far off of the norm, I would probably bring it up to a rep. IMHO, and not really knowing what the color looked like, I would put you in the 8-9 range without a problem. Maybe the guy who gave you a 6 had his card upside down.
Pork,
I agree with the gloppy sauce comment. I also think that the pulled pile could be tightened up a bit. It look just ever so slightly "thrown in". Great looking bark on the pulled. I also agree with the previous comment about the garnish. Try to really make it tight and even. They call it a putting green for a reason. I would say 7-8.
Ribs
Not bad, and it is especially hard to tell because of the red tint. I think the middle line looks too jagged. Try to make it look like it came off of one rack. If that is impossible, then think about a different display technique, maybe stacked. Same garnish issue as pork. Again, 7-8 depending on color.
Brisket
First, I think that you should have put slightly less pulled in the box. I think it looks better if there is a bit of green showing all the way around the box. It helps to frame the meat. Having said that, I think the pulled meat itself looks really good. The slices seem a bit haphazard. The back three look great, but then it looks like you reversed the front 4. Try to stay consistent with the lines. I have seen people cut the ends off so they can make two flats line up. I also like to go ahead and trim the front slice so that it has a nice, neat bottom edge. The jaggedy line does not look as nice. Also, watch the crumbly ends, like on the left side of the front 4 pieces. Same garnish issue. I would probably give it a 7.
All in all, great job!