Very light spoilers ahead....
The directors of Game of Thrones have said many times they were looking forward to Season 4, and dreading Season 5. Here is why.
By my count, there are currently nine plots running in the series, a "plot" being defined as a cluster of characters in a single location. The three major ones are the King's Landing / Lannister plot, the Jon / Wall plot, and the Daenerys / Slaver's Bay plot. Six smaller plots (Stannis and Company, Sansa/Littlefinger, Arya/Hound, Bran and Company, Brienne/Pod, and Ramsey/Theon/Yarra) round out the story. To film the series, the directors tend to focus on one of the three "main" plots each episode, filled with smaller scenes from some of the minor plots or the other two main plots.
As you can see, though, this breaks down in Season 5. If the books are followed, not only are two new significant plots born, but one of the main three splits into three and births another, one splits in two, and the other combines with an existing minor plot and then proceeds to split into five separate lines. At peak levels, there are many as seventeen separate plots going on simultaneously. Two of these do combine towards the end of book A Dance with Dragons (Book 5), and it is clear that many are converging early in Winds of Winter (Book 6), but as it stands, even with some significant pruning of characters and plots, there are some serious challenges to filming Season 5, as so many characters get divided and scattered to the winds. It will be interesting to see how the directors handle this challenge.
Note that this near-doubling of the plot lines is what underlies most of the criticism of the books A Feast for Crows and A Dance with Dragons. With that many threads going on at once, the pace of the overall plot seemed to slow down considerably.
PS: I'll give a small prize to the first person who can correctly decipher my graph!