Prior to visiting Iceland I was always torn between whether to visit in summer or winter. Having now visited in the two extremes (middle of summer and middle of winter) I'm going to outline the most important things to consider when deciding when to visit.
Iceland has the most unique landscapes I've ever seen. Lava fields, glaciers, volcanoes, hot springs, waterfalls, and so on. They all seem to flow seamlessly into each other: if I believed in God, I'd consider him a fantastic landscape architect, but isn't it even more incredible that these sights are entirely natural wonders, and a testament to incredible power of our world?