We'll see what changes they make to the end game from the closed beta. My biggest issue with the closed beta was: What is my goal, now?
Is the goal just to keep leveling to 100? For someone in their mid-30's, that is not something that keeps me playing a game. I would jump back into D3 every season, because I liked the challenge of completing the season journey. I know they will have seasons in D4, but really hope they also introduce the idea of the season journey challenges.
The end-game was pretty much several ways to gain more loot, gain more exp, and unlock harder versions of the dungeons. This gave you levels to apply to your paragon boards. You chose the paragon board based on your build. However, the loot was all just higher level iterations of the same thing you had before. There is a lot of searching for a particular legendary power that you can then remove and apply to a new item. These were all extremely imperative in your build's ability to scale as you level.
I actually stopped playing about a week before the end of the beta because I was getting bored with it.
I also halfway liked the dungeons, but also found them lackluster. If you are going to create a static number of dungeons, which are always the same layout, then add more customization to them. Make each one feel very different or add some "awe" moments when exploring them. There are usually 2-4 sections of a dungeon, which you have to unlock by killing a certain number of something, collecting a certain number of something, or killing a boss. But the dungeons looked procedurally generated, though they were always the same when you reset them.
Combat was great and fast, which I really liked. Frost/Ice attacks were OP in PvP and was the best form of CC.
Also, though there are seemingly several ways to find loot and gain exp, the nightmare dungeons were imperative for you to do for rewards and such. But, like I said before, you were literally recycling the same layout and dungeon over and over again.