A great post here, from the very wise Cheryl Klein, about avoiding plot contrivances. In short: make sure you don't set up plot points that are too-easily resolved.
I see this a lot in the queries I receive, maybe especially in romance novels, which are of course by their nature somewhat formulaic. Done well, they're nowhere near as formulaic as non-fans tend to think, I hasten to add, but of course you need the central focus to be on a love story, and it needs to end on a happy note, either "happily ever after" or "happy for now." But then where would be your novel, without some conflict standing in the way of the happy couple?
Here's a shorthand tip: if the so-called conflict could be resolved with a single conversation, you're probably not thinking big enough.
What's the meanest thing you've ever done to your characters to stave off a happy ending?