The 2nd edition of Advanced Dungeons & Dragons came out in 1989. I was at university, and those years probably constituted the peak of my previous dungeon mastering activity. Even if memory fades it is hard to miss the fact that times have changed. Fortunately many things changed to the better, but not everything.
One big factor, and obviously outside a discussion of game systems, is that the environment for the game changes. 20 years ago I had lots of time and very little money, now I have a lot less time and my disposable income has gone up significantly if you compare it to the price of game books and supplements. These days I consider spending $10 on a poster map reasonable, 20 years ago I would have drawn that map by hand. Of course technology changed a lot too, 20 years ago I didn't have access to software drawing beautiful maps in color, nor the option of sending the map via the internet to a poster printing shop. And I didn't have a fancy color laser printer. So, apart from only playing every other week these days, I'd say the environment changed for the better.
On the game system side I think most changes to Dungeons & Dragons have been for the better as well. The rule system these days reads as if it was written for a game called "Dungeons & Dragons Tactics", but previous editions didn't have all that many rules dealing with situations outside combat either. And I like tactical roleplaying games like Tactics Ogre, Final Fantasy Tactics, or the Disgaea series, even if they have mostly been released on consoles and not the PC. 4E combat is tactical, a bit more difficult to set up than in previous editions, but far more balanced, and overall more likely to result in fun fights. I still remember 2nd edition combat, where when asked what he wanted to do, the fighter inevitable shrugged his shoulders and said "I hit the monster with my weapon", because he rarely had any other option. The 4e system is much better in that respect. And in a way the tactical combat is a return to the roots of D&D, which started its life as a medieval miniatures wargame called Chainmail.
Where I think that a similar return to the roots has occured to the detriment of the game is in the official adventure modules. I bought a number of them now, because I thought I could at least always use the maps and some encounters. But when reading through these adventures, I am very much reminded of the bad old days of the early AD&D 1st edition adventures, with huge dungeon crawls through places that just don't make any sense. Over time the AD&D adventures got better, with adventures offering more coherent places, more roleplaying, more non-combat solutions to problems, and a lot less hack'n'slay. But these improvements appear to have been reversed again, and we are back to pure hack'n'slay with very little story behind it. Maybe it is just nostalgia for the "good old days" of 2nd edition, or the cynicism that comes with age, but many of the official adventure modules I bought I would consider completely unplayable as such. Weak story hooks, no story at all during the dungeon crawl, and forgetable villains at the end. And the adventures you'd think would fit together to form a campaign (e.g. H1, H2, and H3) in fact only have extremely weak connections between the adventures, like finding a map leading to the next adventure in the pocket of the slain villain from the previous one.
So, unfortunately not everything has become better with time. But in the end the weakness of the official adventures solves another problem I didn't have 20 years ago: Players able to look up your adventure on the internet. As I will have to massively rewrite any material I use, there is less of a danger of somebody finding spoilers on the web.