I still don't get it why Doflamingo was introduced so early in the manga, his introduction is almost as old as Blackbeard's introduction.( even was in 5 arcs, was he?) He's not even the main warlord villain, The new warlord could be even be better. It's clear the yonkous are the main villains( except Shanks) in the new world along with admirals but DD's hype was just crazy.
Unless he will be a very important ally in the future( not just in the kaidou arc), all the hype he got over the years doesn't look justified.
If you look at Doflamingo in just the perspective of overall strength, then he does look a little bad. Though I always question the hype that was gathered around him. At most I was interested in who he was and what role he would play, but I never got so hyped that I believed he would be one of the greatest forces ever faced. In earlier parts, we learned he was one of the strongest, or at least had the greatest bounties at the time. We had a hint to his power as well. Later we learned he had dealings in the slave trade. Then during the war, we saw the side of him when it came to fighting, where he gets into it and just has fun with it. There was some other things here and there, but nothing screaming out to me. Black Beard has had much more hype from the few things he has done.
That said, we have to look at Doflamingo less through the idea of strength, but more of relevance of the overall story. He's always been shown before now to be part of some kind of business. He was one of the few Warlords actively going to the various meetings with the Marines. We know Kuma went because of his background, and Mihawk only went because of Zoro. After the war, we see Doflamingo telling that one Marine to shut up, threatening him that he would leave and not care. From there, we can see that he had quite a bit of dealings with the WG, which in this arc we can see why he would keep a close relationship with them. His status his based on information he knows, and his presence can be there to remind them of that fact. This one piece of information will determine his fate after this arc.
After learning about his dealing with the slave trade, we later learned about him having a whole underground trade, dealing in weapons of various kinds as well as smiles. He also had a hold over a kingdom, and held other information pertinent for other parties. With him now down, all the trade is gone and open to the public. Kaido no longer has smiles coming to him, which seems vital to him being an Emperor. Their are groups that seemed to rely on weapons being distributed for who knows what reasons (we know for a fact Kidd had dealings with him, and that will come in play). We also know that many wars against countries have been continued by Doflamingo supplying both sides. For good or bad, everyone will be thanking/blaming the Luffy/Law alliance for what happened.
Last but not least, we have what is the start of a shift in the WG, which is the foundation of abolishing the Shikibukai, but also the WG image falling apart. All the others before were covered up and deemed inconvenient at most. With Doflamingo dabbling in all these practices, which was possible due to his position, questions can now arise about if the position is a good thing. Unlike others which were doing their own thing, Doflamingo was doing many things that would go against the WG. That would be bad enough, but we have something that will shake things up more, and that's this incident not being covered up, along with the events being amplified by Fujitora's theatrics. Citizens are going to start questioning the WG, which can help the Revolutionaries cause. Then there's the Marines, which will eventually start being fractured by people of certain beliefs not wanting anything to do with a government they deem corrupt.
In the end, Doflamingo's hype was less about strength, and more about his role in the overall story and plot. Even then, one can argue that he was still impressive. He took a number of hits from Luffy and Law, and was able to fight quite well after taking the Gamma Knife. After that, he still had to force Luffy to show his new gear. He took a number of hits and was able to keep going, and it took a King Kong Punch to finish him, which by all means should've killed him. I know some wanted more from the fight, and even I felt it could've been better, but if you look at it from a story perspective, I think it worked out very well in the end.