FunkyFr3sh: I think blizzards anti-cheat won't allow the use of ddraw proxies in online games (aka the gog compatibility layer) hence why they made 2 versions. One which will probably not work too well on modern systems but is capable of playing online games and one that should run fine on 7/8/10 but can't be used online due to the anti-cheat in battle.net.
640x480 might not always be supported, hence why the gog updated version probably is scaling the game to a higher resolution (But the actual game still runs at 640x480).
This is not the case. First, there's no anti-cheat measure against the ddraw patch and I can connect to the new gateway with the ddraw patch without any issue.
Second, the classic version also uses a ddraw patch.
The only reason they made 2 versions is that the "improved" ddraw causes all texts in the battle.net UI to move off-screen, which makes the battle.net interface unusable in fullscreen mode and very annoying to use in the windowed mode.
Resolution also isn't an issue. You can still run the original Diablo 1 natively in Windows 7 and Windows 10 with some tweaks (terminating the explorer process for example), without any ddraw patch.
High-resolution support simply means that the game can run in whatever resolution your monitor/GPU can support (4K?), but it doesn't change the fact that this game was created in 1996, so there will be pixels and there will be 4:3 window.
Finally the Beelzebub mod that most people label as "HD mod" is no different from this release in terms of high-resolution support. It is in fact identical in that regard!
The difference is that Beelzebub changes two things:
The default zoom distance is more zoomed out, therefore the pixels look less pixel-y, because you're looking at them from afar. But they are still the same pixels as before, there are no remastered graphics involved, and you can check that by zooming back in. The default 4:3 renderer frame is changed into an adaptive one, which results in your screen not having black borders no matter which resolution you use.
Both of these changes are changes to the game code, and I'm not sure if they can be added simply as a patch, perhaps Mordor can enlighten us on that topic.