I'm going to talk about text-related (not writing-related; that will come much later) problems first, because they're incredibly common, and also simple to fix.
There is one overriding rule when it comes to text, and it is this: if I can't read your text, I'm not going to read your comic.
So don't make your text too small for me to read it without squinting. Don't put too much text in a single balloon. Don't shrink the space between lines so you can cram more text into each balloon. Don't draw your balloons right up against the edge of the letters.
And for the love of god, don't do this.
Text should be big, clear, cut into small chunks for easy digestion, and have plenty of breathing space both between lines and around the edges. Like this. See how easy that is to read?
A Word About Fonts
Arial, Comic Sans, Times New Roman. DON'T USE THESE. In fact, don't use any font that came preinstalled on your computer. Why? Because they're ugly. They're also hard to read. They were never meant to be used in comics. They look amateurish. But mostly, they're just ugly.
There are a variety of free fonts specifically designed for use in comics, readily available from sites like Blambot. Pick a good one from their list-- NOT "Anime Ace", that one looks like crap too-- and use that instead.
A Word About Linespacing
If you letter your comics in MSPaint, then close your browser window right now and walk away. Your comic cannot be saved. It is DOA.
If you letter your comics in a proper image editor, though, you probably have the ability to tweak something called "linespacing."
This is how you do it in Adobe Photoshop (although the same technique applies for Illustrator):
Once you've got your text banged out, hit Ctrl + T to bring up the Character palette. The little icon with two A's on top of each other stands for linespacing.
Most of the time the (Auto) setting is good enough, but for maximum readability you may want to dick around with it. I prefer to err on the side of too much space between lines rather than too little, so I usually set linespacing to be a size bigger than my actual font size. In this case, my font is 18 point, so my spacing would be 24.
That looks better. But remember, not all fonts need to have their linespacing screwed around with. Use this technique sparingly, and use it for good, not evil. NEVER EVER use it to cram the lines closer together so you can pack more dialogue in.
That's all for today. Next up: balloons.