Discarding the preprocessor and replacing it with a proper module system is huge. I got burnt by templates and horrifying compile times in C++, but haven't had any problems with D templates. The module system makes templates feel much more natural to use. The syntax for templates is a huge improvement, and throwing `static if` into the mix results in concise and easy-to-read code.
I also quickly realized (with the help of some people on the D discord) that the garbage collector is fine for my needs. So I don't have to spend any time thinking about memory management... put stuff on the stack when I can for speed, othrewise just GC and don't think about it. I think there may be some issue with multithreading and the GC, but this is supposed to get fixed with the new GC that's on the way.
There are a few other nice QOL improvements. Getting rid of `->` is honestly worth its weight in gold. There's nothing difficult about forgetting to change a `.` to a `->` or vice versa in C++, but not having to trip over it periodically when you're compiling makes the language that much smoother. I was also initially confused by the `inout` keyword but have come to really like that, as well. Little niceties like `const(T[])` are small but, again, reducing just a little bit of friction like this across the language makes D much, much more pleasant to deal with than C++.
I think the main challenge the language is facing right now is that it's huge and a lot of it is still getting worked out. I never thought I'd pine for C++'s "rule of 3/5/0", but it's a lot tighter and more logically consistent than the equivalent in D. But part of that is there being a huge community of C++ developers who have taken the time to promulgate rules of thumb in the community. I'd kill for an "Effective D" book to short circuit some of this process... after all, I'm trying to write code, not play at the margins, tinkering with D's idiosyncracies.