I've been doing a little bit of contest judging recently and have come to realize a few things that I may have already known, but just didn't know I knew them. But even though they seem obvious, they are a good thing to keep in mind--especially if you are having trouble getting agents or editors to ask for more pages.
Start with tension--any navel gazing introspection or flowery setting descriptions or back story dumps needz to go.
You NEED a premise and that premise NEEDS to be fairly self-evident (that does not mean blatantly told, it means self-evident) from the get go--not by the end of chapter 3.
I think the majority of the entries I've judged have had this down, but the ones that don't leave me with a totally lost feeling. As a judge I don't get a blurb to set the premise up, and some of these contests don't require a synopsis. But really, your opening pages should not rely on a back cover blurb or synopsis. Don't presume that an agent or editor will be reviewing your blurb again before reading requested pages--they won't be, they'll be diving right in.
Another thing that stands out for me is voice. Voice is not about dialog or (gasp) dialect. In fact whenever you hear voice, you should immediately think personality. How a character views the world should match how they tell us about that world. A shy person will see things differently than an outgoing one. Catch phrases and the like are fine for a character if they match their personality. But don't rely on them. The bigger point about voice is--if you are going to alternate 1st POV you better have two or more characters with very different personalities. Otherwise you end up with everyone sounding the same and if you have any voice at all it will start to make the reader think of the author and not the characters. Bad.
So quick recap:
Start with Tension
Premise is self-evident.
Characters speak. Author types.
Remember those things and you'll do well!