Writing a spec screenplay is difficult. Writing a spec screenplay that someone will say, "Bloody hell! This is brilliant! I'm going to pay this scribe well and not change this concept beyond recognition!" is akin to making it up Mount Everest on rollerskates. You may make some headway, but chances are you'll slide all the way back down and collapse in a bloodied heap.
Any movies that are made of your script will be changed drastically from what you first conceive. That's the fate alloted to all scribes. Why fight the inevitable? With a bit of luck, the movie made of your script will resemble what you came up with in the first place - albeit with a load of extra stuff put in and a bunch of stuff you really thought was cool and/or important cut out. And oh yeah: let's not forget others' interpretation. You saw that scene as green and the set dresser's gone for yellow. You saw that character as black yet the casting agent's gone for white. Oh and...those lines you thought spelt out the whole ethos of this story suddenly sound as wooden as the table in your living room.
I think of dialogue as the area as in need of least attention in a spec draft (notice I didn't say least important). Why? Because even if your beloved spec does get optioned, that's the first thing that's going to go. Why? 'Cos messing around with plot points when the director says, "Wouldn't it be really cool if..." demands it. Or they don't have enough money to do this or that or the other, which means whole scenes may have to go. If the scenes go, the dialogue goes -but crucially, anything you've revealed through dialogue in other scenes also has to go. In short, sort out various things (like structure), and dialogue *generally* falls into play with them.
Yet I notice scribes obsessing over their dialogue. I have had clients who've wanted to keep even the most flawed chunks of their scripts on the basis of just one "clever" line. Whilst on-the-nose stuff is never desirable, in early drafts who cares exactly what your characters are saying to each other? You'll change it later and even if it gets sold, it'll get changed again... And as soon as it's spoken by the actors it will change even more! What's more, I have never, ever plastered a "consider" or "recommend" on a script that has only good dialogue. Yet I have put through scripts with bad dialogue if they have something like good structure and/or an original premise. Here's why.
There is no such thing as "nailing" dialogue as far as I'm concerned, just the best approximation you can make in any given draft dependant on where you are with it. The reason for this? If your script is a house, then perhaps a spec is a house of cards, with the dialogue cards aas its roof. You can take them off and the cards won't fall down. Take the ones at the bottom though - the structure cards - and you're screwed my friend. Your draft won't stand up. After all:
You can have the best dialogue in the world, but if your structure sucks, then well--your script sucks. Who's going to buy something where they have to lay the foundations to start with? They might as well write it themselves.
Not to mention--
You can have the best dialogue in the world, but if your scene description is heavy on black, then who's ever going to read on past page 10 and find out?
You can have the best dialogue in the world, but if your characters are stereotypical and 2D, how is it going to resonate?
You can have the best dialogue in the world, but if your arena is non-existent, then surely the same goes for that notion of resonance?
You can have the best dialogue in the world, but if your story is ill-advised, boring or unoriginal, then why would anyone remember that dialogue?
You can have the best dialogue in the world, but if no one does much in your screenplay you might wow a few people with your observations, but does that mean your script is an interesting read or top heavy on telling it?
You can have the best dialogue in the world and whilst we might remember snatches, is this the only thing we watch films for, or the fact that we can relate to characters, stories, ways of life?
Dialogue. It's important. But not the be-all and end-all as far as I'm concerned.