Exposure is a critical key to a great photo. For me it's what makes or breaks it. When taking landscape photos pay close attention to your exposure. Make sure the clouds and sky aren't blown-out (overexposed), and that your foreground is still bright enough to where you can make out details. The key here is to strike an equal balance between detail in both. You know you got it wrong when the sky is glowing so bright that it starts to soften edges on the horizon.