I think Diablo 3 is great. However, there are indeed issues with it. I started playing about 1 week before Season 9 came out, then jumped right into a Season completely lost. I learned quite a bit about the game and now feel that Season 10 should be far better for me.
For this season I managed to clear a GR80 on both an Invoker and a LON Bombardment build (Both Crusader) and a 70 with my Wizard, though the time spent on the wizard was probably less than 8 hours. As soon as I finish augmenting, I'll be attempting an 84/85 on my Lon-Bomb Crusader before Season 9 ends. I got my portrait a few days ago, which was the final challenge for the season.
Here are some things I really dislike:
1) Takes forever to farm the materials to 'reroll' items. About 2/3rd's through the season, I took a stab at joining multiplayer bounties and wow, that was great. While somewhat tedious, it is far less tedious to do bounties with a group. Patch 2.5 is going to make it so that the order doesn't matter. Less people to screw it up, more materials, of course. Still, it takes a lot of bounty materials to reroll decent gear (even more if you want an Ancient with good rolls)
2) Greater Rift fishing. You look at the leaderboards and think "Wow, how are these guys 10 or 20 levels ahead of me" and without know any better, you just assume they are better. However, there are two main reasons for the difference, besides skill. A) Rift Fishing B) Paragon Farming. Though, this isn't to say the players on the leaderboards are not skilled, they are. They are definitely skilled to make it up there, but the majority of the difference between them and you is fishing and better gear/paragon levels. You can even look at their gear and they have perfect rolled ancients, augment with 100 gems of mainstat. I would much rather the leaderboards show 3 consecutive rifts completes successfully. This 100+ attempt fishing is ridiculous and entirely based on luck. Make it three in a row and then we know exactly what a build or character is capable of.
3) Greater Rift Variability. This is the reason for the fishing, but ultimately, GR can be really, really, bad. They sometimes will have a level with no elites and sparse white trash that, even killed, does not really move the progress bar enough to make it every worse killing.
4) Mob Progress points. Killing a huge pile of trash is a huge question mark. On my lon-bomb I kill piles and piles of trash, some moves the progress bar, others barely at all. No reason either, some of the mobs that hit harder and have more health have the fewest progress points, while fat slow melee mobs give some huge progress. Honestly, it is clear to everyone just how stupid this is. There is definitely no algorithm for determining progress points from the devs. It was very clearly arbitrarily picked by the devs... There are websites dedicated to showing you how many points each mob gives. Well, TBH, I am not going to memorize a list of 100+ mobs so that I can know which mos to skip and which ones to kill. I kill everything, typically. But if I did skip, that would be
Basically all my complaints are with the leaderboard and how greater rifts are too random and non-nonsensical. Randomness is fine, to a point, but it is uncontrolled and stupid.
That said, I love the game.