Hardware accelerated OpenGL. As far as I have been able to tell, that will still be missing if you run an aarch64 kernel.

And I think using the GPU to get hardware accelerated video decoding and encoding will not be available either.

Edit: But if I understand https://github.com/popcornmix/omxplayer/issues/714 correctly then you could do hardware accelerated decoding of HEVC on the CPU. I don’t know how the performance of that compares to the kind of video decoding that the GPU can do. That’s one of the things I’d like to see someone benchmark, or benchmark myself.

That was the case but the VC4 instruction set is open/documented and there is a Mesa driver worked on to REPLACE the closed binary driver which is only available for 32-bit. When you use the Mesa VC4 driver you can also compile the whole user space for 64 bit including OpenGL ES support in a even newer version then the closed source one. (Supporting user kernels etc.)

You can find some info about that here: https://wiki.gentoo.org/wiki/Raspberry_Pi_VC4

Some side notes: Mesa is quite hungry in performance and memory to compile shaders (to VC4 instructions), way more then the closed driver requires, thats why older versions of the Pi with less powerfull ARM cores couldn't really use this approach.

Source: I used to write custom user shaders in VC4 assembly to run on all Pis because the closed binary didn't offer OpenCL.