This is my list of pet peeves!
Ok, From the bottom up:
5. "To be continued..."
It's an unfortunate side effect of working in small press that sometimes your ambitions over-reach themselves. Now usually this is a good thing except in one tiny area: the horrible 'to be continued'. If it's your first small press title then, for the love of god, don't make it an on going strip. I've lost count of the numbers of issue #1s of comics I have were the promising series didn't get beyond, well, beyond number #1.
Ok, you have to work A4 - fair enough, but try to make the best use out of the space - have a look at a Commando comic, 2000AD and any US comic and you'll see that the number of panels grows and shrinks with the page size. Don't try and make a US comic at UK sizes - it just has the horrible effect of making the whole thing look overly large (and we all want our cool things to be smaller, right?).
3. Rubbish Reproduction.
Well, this could mean a lot of things, but really, what I'm getting at here is rubbish reproduction which you can do something about - ie, if you're doing a digital print, try and make sure the work is at least 600dpi at print size - smaller than that and you will start to notice jaggy bits.
Don't include COLOUR! It usually pumps the cover price up and a bad colouring job can really make a comic look amateurish. in particular don't use either of the following in photoshop (or whatever your computer tool of choice is):
Lens Flare (ARRGHHH MY EYES! MY EYES!) or the air brush tool. Both have an instant 'Yuck' factor.
And finally, and, perhaps, the most importantly, lettering. Now I'm really rather of the opinion that lettering is FAR more important than almost any other factor (art and script included). Good lettering is a joy to behold and will instantly elevate your work - bad lettering will do nothing but totally detract from everything around them. The quickest way to spot quality is to look at the lettering. And you know, there are fonts other than COMIC SANS (for the love of the sweet zombified baby jesus, NOT comic sans!) - and they're not that expensive and will last forever.
(Oh, one other tip on lettering: whatever size you're lettering is? shrink it by about a third.)