Djokovic wouldn't have been playing nearly as well were it not for that ~2.5 year gap. Two major reasons:
1.) His messed up serve forced him to become the best returner of this generation, just so he could keep the matches competitive. That amazing return has remained to the present. And not just the return, but arguably the movement as well. A superb serve very early in your career is a crutch that doesn't encourage you to develop the other aspects of your game quite as much.
2.) His humiliating losses due to fatigue, poor conditioning, or poor dealing with circumstances forced him to work in a focused way on his stamina. That has proved invaluable for the many long matches he's won over the past year - and especially for clay.
Had Djokovic not had his bad patch and had instead remained on the same track he was on in AO 2008, he may have won 2 more hardcourt slams (say one more AO and one more USO), but he never would become as dominant as he is now, and it's doubtful as to whether he would win Wimbledon or become a prime contender for the FO.
Good points, agree that most big servers have bad return, because simply they never try so hard to develop it when you have great serve, but Nole had good serve and return before, simply his serve is not so lethal but good enough with his groundgame.
2nd point is right point, Those health issues that Nole had, make him think that something is wrong and he found out about that gluten allergy and also i believe he is now working on his fitness-stamina much harder than when he was younger for sure.