First, I saw nothing explaining it in a sticky, and was unable to search for "HD" since the GOG client requires the use of 3 characters or more for searching.

Basically, I am playing on a modern system (I assume most people are) and so I am interested in the differences between playing it in HD vs vanilla, but that depends on what HD means/does. When I play using 640 vs 1080 I see no difference. I do see a little when I turn on MSAA, so what exactly is this game *doing* when you choose to play it at a higher resolution?

Is it simply taking the exact same frames and scaling them up to your monitor, like resizing an image? There is no descipriotn in on the GOG page that I see here (using the GOG client) and do not see an actual answer in the forums yet. Maybe it was discussed on thred #9,999, but no thanks. :/

So, they advertise this as different because it has "HD" but what does that mean for people who play at different resolutions? Any answers that help are very appreciated. Thank you to anyone who can help!

...And Hellfire support, right :)

Hellfire will probably not be put before 1.2.0

I agree with the author of the post. Also did not see the difference.
If high resolution means that you have added the ability to apply filters to the game, then this is strange.

HD in this case is just a clickbait, if you want to see more of the game after increasing resolution, try devilutionX like AJenbo linked above

Try PS1 version. Even with a half of the original resolution, the graphics looks much better. IMHO. It's something like with Biohazard, when PC ports had much better resolution but lacked in all other things.

P.S.: Actually, it's kinda strange that even the excellent PS1 port didn't help in sales in Japan. Maybe because of the largest save files and load times. And this port probably can have a translation in your region. I don't know actually 'cause I played only US version.