Yeah, trust me, I hate the locked hotbar too. Unfortunately it's a side effect of the current skill UI. They correlate each skill unlock to a specific hotbar location which they lock as well. The last iteration simply didn't put the mouse button or hotbar key next to the skill unlocks in the skill window. Which meant your entire bar could be unlocked from the beginning and had fuck all to do with how many assignable skills you had unlocked.
I prefer to assign my skills right to left, so I'll be forced to rebind my hot bar everytime a new slot opens up. Luckily it only takes you to lvl 19 to unlock the entire bar, but again, this is one of those small things where I feel Blizzard has dropped the ball.
The main things I see that just aren't as good as they could be are:
Main screen UI - Can we not get an exit game button on the left side? I dont know, but the arrangement of things here just isn't feeling intuitive or well laid out.
Skill UI - Having to page through multiple categories, which feel quite arbitrary, to find skills that may or may not naturally fall under said category is quite a pain. I really thought as long as I had been in beta that I would eventually "know" which Demon Hunter Skills were where and which category came after each category. This simply did not happen.
Locked Hotbar - Unfortunately a side effect of the Skill UI, pisses me off to no extent and while it's a minor quibble in the long run, it's simply not as good as it could be.
Elective mode - Another compromise made by Blizzard to offer some sort of guided approach to skill selection and build creation. I just wish Blizzard would trust the players enough to scrap this way of thinking. I also don't think that a tooltip and through the grapevine is a very professional way to go about explaining a feature that the vast majority of people will prefer to use. If I'm overestimating majority here, I believe it will be because people won't even know about elective mode.
Hitbox - This one I will mention, but it's not a huge issue for me, but it can be a pain at the right time. The hit boxes are so forgiving on the mobs that targeting the correct one quickly in a pack can be a chore. Again, this one isn't huge for me, but I've seen many complaints regarding it and it has happened to me on occasion.
For those talking about difficulty and character level, I thought this blue post was interesting.
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