Much of what made the 50th special was more than the daily offerings in the park, it was the change in philosophy that came with the event. DLR was coming out of arguably it's worst period ever, with the post-9/11 recession, DCA's failure to launch, and nearly a decade of maintenance and operations cutbacks that had killed multiple guests. Attendance had slumped, management was putting all their attention into quick-fixes for DCA, and DL had been neglected.
That whole approach changed with the 50th. Due to the hasty planning and low-budget nature of the event, instead of featuring splashy new attractions (like WDW ironically got for DL's 50th), the park was given a thorough top-to-bottom series of refurbishments and looked better than it had in years. It felt as though management was turning a new page, and moving away from the decisions that allowed the park to stagnate and decay.
It also marked the first time that the park really made an appeal directly to its fans and marketed its own nostalgia. Instead of simply being part of Disney in general, the event acknowledged that the park was a brand of its own and had its own fans. The whole spirit of the event tied into the guests themselves, and the emotional connections that had been forged with the park through the years. Paired with the explosion of APs (thanks in part to the introduction of monthly payments around that same time), it felt like the first time that the parkgoers themselves were real stakeholders.
Yes, the fireworks and parade were great, the merchandise was neat, and the decorations were classy, widespread, and lovely. But what really made it stand out was how it was a breath of fresh air, putting the bad times in the past. Even at the time, it felt like the start of a new era, which has more-or-less continued to this day.
On the flipside, that same approach was relied upon for too long, often to the resort's detriment. Instead of new meaningful additions, they pushed self-referential nostalgia at every turn. Instead of drawing those once-in-a-generation guests, they've become a local's hangout where infrequent visitors often feel out of place. The last few years have finally brought more meaningful investment in the park's future, but not necessarily the kind that some people would like to see.
I'll also add that I was there on July 17, 2005, and it was just an incredible experience. From spending the night lined up in DCA with thousands of other fans (my first time really being immersed in Disney fandom), to having the CMs greet us "Welcome home" as we walked up Main Street, to the huge rededication ceremony in front of the castle, to the free ears and cupcakes, the energy was absolutely electric. And with all the warnings about how busy it would be scaring away casual visitors, the crowds were delightfully thin and like-minded. It really was one of those once-in-a-lifetime moments that you just had to be there for.