Like iOS 10.3, High Sierra will convert your boot drive to APFS when you first install it -- this will be true for all Macs that run High Sierra, regardless of whether they're equipped with an SSD, a spinning HDD, or a Fusion Drive setup. In the current beta installer, you're given an option to uncheck the APFS box (checked by default) before you start the install process, though that doesn't necessarily guarantee that it will survive in the final version. It's also not clear at this point if there are edge cases -- third-party SSDs, for instance -- that won't automatically be converted. But assuming that most people stick with the defaults and that most people don't crack their Macs open, most Mac users who do the upgrade are going to get the new filesystem.
HEVC and HEIF
All High Sierra Macs will pick up support for HEVC, but only very recent models will support any kind of hardware acceleration. This is important because playing HEVC streams, especially at high resolutions and bitrates, is a pretty hardware-intensive operation. HEVC playback can consume most of a CPU's processor cycles, and especially on slower dual-core laptop processors, smooth playback may be impossible altogether. Dedicated HEVC encode and decode blocks in CPUs and GPUs can handle the heavy lifting more efficiently, freeing up your CPU and greatly reducing power consumption, but HEVC's newness means that dedicated hardware isn't especially prevalent yet.
While both macOS and iOS still nominally support open, third-party APIs like OpenGL and OpenCL, it's clear that the company sees Metal as the way forward for graphics and GPU compute on its platforms. Apple's OpenGL support in macOS and iOS hasn't changed at all in years, and there are absolutely no signs that Apple plans to support Vulkan. But the API will enable some improvements for end users, too. People with newer GPUs should expect to benefit from some performance improvements, not just in games but in macOS itself; Apple says the entire WindowServer is now using Metal, which should improve the fluidity and consistency of transitions and animations within macOS; this can be a problem on Macs when you're pushing multiple monitors or using higher Retina scaling modes on, especially if you're using integrated graphics. Metal 2 is also the go-to API for supporting VR on macOS, something Apple is pushing in a big way with its newer iMacs and its native support for external Thunderbolt 3 GPU enclosures. Apple says that every device that supports Metal should support at least some of Metal 2's new features, but the implication there is that some older GPUs won't be able to do everything the newer ones can do.