The delay will be due solely to the number and complexity of the visual effects shots. That's the reason the seasons are written the way that they are, with the heaviest VFX episodes left until the very end. Given, as firesprite points out, that the two armies of the living and the dead will soon to start warring in the North, I imagine the next season will be how the Seven Kingdoms come together , who gets the dragons, with possibly the wall falling at the end of it, followed by the last season which will be the actual battle in the North. So I imagine we will get some new things like dead mammoths and giants, maybe some ice creatures to combat the dragons fire, which will take a huge amount of time to get done. Remember, we are talking seven hours of screen time, not a two hour movie, so to do it justice they have to take their time with it, which is fine by me. Every year I watch the entire series before the new season is released and (with the exception of the first seasons dire dog wolves) the sheer high quality consistency of the show is astonishing strong. To have maintained it with the same cast (well ,be it an every changing one I guess) is nothing short of a miracle. Give them the time to deliver the brilliance that we have come to expect of them.
