Recently dusted this sucker off and started to play. I love killing the Butcher, Zhar the Mad, and the Warlord of Blood. I'm at level 14 (second level of hell), and am a level 20 Warrior. I love Flamewave, I wish they would've had that in Diablo II. You all should try it, if you haven't already. System requirements, well, you basically need a computer that was made after '98...
Diablo 1 is the shiznit solo. I wish it were more multiplayer friendly, but unless you're playing 2 warriors, you'll end up killing each other. Rogues arrows hit friendly targets, but that's somewhat avoidable. Sorcs will just kill anyone and anything on the screen with most of their spells.
Apocalypse pretty much explodes every monster on the screen, damaging them all pretty heavily.
I think the only way you can use apocalypse legitly in D1 without the hellfire expansion is to use a scroll or staff. In Hellfire, I remember you get a book of apocalypse eventually, but that only gives you level 1 apocalypse.
The level 1 version of apocalypse isn't all that great really. The only real bonus of it is you can hit shit that you can't even see, like if there are a bunch of monsters behind a wall or something. It literally hits everything on the screen regardless of anything.
Is there a way to download Hellfire? I've been searching for it, and it seems that I would have to purchase it on Amazon or something. Not sure if it is just a retail item. I had Hellfire on my old computer, but it is gone because the harddrive got fucked by a virus. (Only backed up important documents)