1. Proportianate. A simple issue that can ruin the entire look, no matter how detailed or neat your costume is, if you have too small weapons or way too large knee pads - that is quite a serious issue, so it is worth taking a time in the beginning to scale your character down according to your size and getting these proportions right before you start cutting your materials.
2. Ironed. This may seem silly, but I have seen lots of cosplayers on stage with un ironed cloth pieces. I have no justification for this, and "not having an iron" is also not really an excuse, because I am sure you can borrow one, and you can take your costume to the con on a hanger. Ideally you should choose fabric accordingly to how messy it gets after wearing or handling, and stick with something that does not require ironing every fifteen minutes or so. Iron is also a must have when you are just sewing a piece, for example, to iron out your patterns at first, then to iron out the fabric before you even cut it, to flatten out the seams in the inside of the garment, to get the edges of bias tape nice and flat, to heat on appliques, to make the fabric more sturdy with fliseline, and a thousand more uses. Actually, me and my very good friend and collegue at work have this saying "if faced with a problem - try using heat". It works both in a sewing department and in prop making (heat gun), I have solved thousands of problems that way :))
3. Your seams should be finished on the inside. That's pretty much self explanatory, when looking on the inside of your seams, I expect to not see fraying edges.
4. Your costume should have all the details that are in the reference. This is rarely the case, but generally the more details you will have transfered, the better for you. I will absolutely notice them all :-) I will also give bonus points if you can take of a part of your costume and underneath there is still an accurate costume, and not a hidden tangled mess.
5. You should have more than one or two techniques in your costumes. If your favorite material and the only thing you work with is EVA/WORBLA/SINTRA (insert any), try and make some details using something else, e.g sculpt them in polymer clay, carve them out of xps, cast them, and so on. The more techniques you will have in your costume - the better, this is the thing that EC judges actually look for.
6. There should be no hot glue or any other glue clearly visible on your costume. Also, pretty much self explanatory so I don't think it is needed to dig deeper into this.
7. You should have neat seams and surfaces. Now I am not really talking about the clothing seams, of course they have to be neat, but there may be required to have more seams than your reference does, because for example an artist just draws a naked character in anime and then draws on some clothing, obviously that will not work in a real world, so you need to make seams for a fitting garment in quite a few places. This is not the case for the props! I understand you need to make this part out of a few pieces, because you cannot bend the material that way from one large piece, I do this all the time and I understand it. But I do not understand when the prop is finished and painted, and I can see the extra seams has not been filled out. Or I can see them overfilled and sticking out. It is supposed to just not be there, guys, you can make thousands of seams while making your prop, but the final piece has to have just as many as your reference does. The surface can be smooth or it can be textured, depending on your character, but it should not have your fingerprints and other things that do not belong there :)
8. Your paint job should make sense. Now that is something that I would like to see more often than I do.Things to consider is not only the right colors, but whether you need to have a color solid and glossy, or matte, maybe darker to the edges, maybe two toned colors, dark or light undertones, etc etc. Look at your reference and see more than just a color. Also, I think that artificially painting on highlights with something white-silver is really a bad idea for a 3D object (with some exception in cartoony character cases), it will look awkward and obvious from more than one direction. You should aim for natural, blended highlights, that will actually bounce the light of your prop. Metalic surfaces: do not use metalic paint spray paint cans. They have thousands of shiny particles in them, sort of glitter like, and your sword blade ends up looking like a brand new silver colored Toyota car.
9. You should have weathering. I guess I must say what weathering is. There is a huge common misconception that it's all dirt, grime, and battle damage, and if your character is "clean" and "noble" he will have none. That is not truth. If your armor has edges, there will always be darkened seams right next to them, because if it catches any dirt ever, it will be impossible to remove. There will always be something in the areas that a hard to reach and impossible to clean, so think about that. It is incredibly obvious to me if a costume has or has not been weathered at all, so a little goes a long way. Just make your character "real" and not a toy figure, that's about it. Think about who your character is, what elements is he or she (or even it) exposed to, how old are the pieces that are being worn, which era are they from, and many more.
10. You should be happy and proud of what you did. It is too late to think what you could have done better when you are standing in front of the judges, just defend your costume as best you can! ;-)