The game is not perfect by any means. It is just no where near as bad as some people are making it out to be.
Improvements could include: Group finder, better chat function, faster travel, reduced cost of respecs since we are all just learning them now. Ability to turn off PVP. I have never been much for PVP and there are areas I will not venture into, especially in HC, just because of the griefers. Last thing is cooldowns. They seem excessive to me.
What I like so far is: Campaign, story, graphics, open world events like world bosses, and especially strongholds. I also love that you need to kill a particular boss in order open up nightmare and torment levels. I think that is better in D4 than D3.
Love/hate things: itemization, just because I need more time to understand the synergy between different aspects and afixes. Number of afixes too, but just because I don’t fully understand how they all work yet. I think once I understand these things better, i will love the character customization more compared to D3.
Missing things that will probably be added in future expansions: crafting. I am hoping the reason it is not in the game yet is because they are still working on it and will provide the ability to create very specific custom items, not just random afixes.