Hearts of Iron 3 was a great game with some flaws. But in my opinion, the greatest flaw of all was the diplomacy system--a mere shadow of what could have been the greatest realpolitik simulator.
To me, this makes only slightly more sense than saying that HoI3's greatest flaw was the lack of first-person vehicle controls that could have made it the greatest tank simulator. That's not to say that a game with more developed diplomacy wouldn't be interesting (just like a tank simulator can be interesting too), but I recognize that game development teams have limited resources and I prefer my wargames to focus on war-related features rather than add non-core stuff like extensive diplomatic options for shifting the major game alliances (that's more of a 4X kind of feature).