Start with design. It's a tank that outweights PzIV(H) by 20 tons (44.8 tons against 25), T-34 by 12 (32.2 tons for T-34-85) and Sherman by 14 (30.3). Germans could call it like they wanted, but it wasn't a medium tank to start with.
Is it a miracle that it had superior armor?
But add two more tonns and you've got IS-2 that had better armor protection, better gun (75-mm of the Panther was more like dedicated AT, 122-mm being slower but stil more versatile)
Weight isn't the only thing to consider, it weighed a lot because it mounted a large gun and had excellent armour protection. It also was only slightly less mobile than a Sherman or T-34.
And I wouldn't compare the IS-2's gun with the Panther's. The Panther's gun was designed for AT, it fired a 75mm high velocity shell designed to defeat any equal tank on the battlefield. The IS-2's gun was designed with 'breakthrough' in mind, utilizing powerful HE Shells. The IS-2's AT rounds had similar penetration performance to the Panther's, but was notably much much harder to reload. The massive gun had to be set to the neutral position (meaning each shot had to be aimed individually), and the large two-piece shells were heavy and difficult to load, leading to a much slower rate-of-fire than the Panther, and undesirable in a tank battle. The IS-2 was meant to shatter the defenses for the much faster and more versatile T-34/85's to exploit, while the Panther was meant to combat other tanks including heavy-hitters like the IS-2.
Add the fact, that Panther's design left no place for any upgrades(check how PzIII and Pz IV evolved). So afterwards Panther was intended to be replaced first by Panther 2, then remember E-series E-50 project.
The Panther II and Entwicklung series are completely unrelated to the Panther's supposed inability to be upgraded, which isn't true. First off, the Panther did get upgrades over it's life-span, the very fact that a Panther Ausf. G exists is proof. The Panther II was a heavier version of the Panther. At Hitler's insistence, it was to trade speed and mobility for armour and firepower, but never came to fruition and only one chassis was built.
The Entwicklung was a completely separate project to simplify all tank production into one standardized set, which would be much cheaper and easier to make and maintain. This never came to fruition either due to the rapidly deteriorating situation on all fronts, and only one partially completed E-100 chassis was made.
In the end I'm not saying the Panther was perfect, like coh2player said, it had reliability problems like much of Germany's armour. The entire reason the E-series was devised was to solve this one-and-for-all, but for Germany, the solution was far far too late.