Was thinking about making a hardcore guide... played the last 8-10 seasons hc..
Things to watch out for:
1) Cold/fire/lightning enchanted boss monsters.
2) Unravelers in act 2 tombs, especially champion packs. Their unholy bolts deal big damage and put you in stunlock.
3) Boss battles is typically dangerous and as others write, resistances are vital. You can use thawing potions on duriel, mephisto and baal, on you and merc. Make shield with diamonds/topaz/sapphire. You can use the moat trick on mephisto.
4) In nightmare, things typically becomes more difficult. The tainted-type monsters, that shoot lightning orbs, are the ones to look out for. Also scarab demons that emit lighthing bolts when hit. High lightning resistance is almost vital to avoid taking big damage constantly.
5) If playing d2l, quest bugged diablo/baalruns is a great way to build exp and items, also stockpiling gear for if the worst should happen, for new characters. I consider it almost essential. Diablo is great for easy route, baal is great for big exp boost.
6) There's zero point in venturing into hell mode until you got at least medium-high end gear and have a high level. You will hit a brick wall, not be able to make any progress, and just about anything will almost obliterate you. Your merc will die constantly. Especially the packs of quill rats outside rogue camp is a vicious trap for unsuspecting hc players. Their bolts are hard to see and can hit from outside the screen, if they have extra strong+might or similar, they can one-shot you. Even if you have good gear and high hit points they are a threat. The only way to make progress underprepared is to single out monsters and deal with them one and one. And that takes a lot of time, and is not fun. You will not be able to farm bosses effectively either.
7) You can choose to play two ways: a) To breeze through, trying to avoid the big dangers and hope to survive, accepting defeat when it happens, or b) Identify any danger and play to survive, playing the way it is meant to be played.
8) The regular dangers; Black souls and stygian dolls, you have to prepare for before facing them. Stygian dolls can be unforgiving and unfair, they can desync, and any lag can make them appear next to you and thus be a threat before you can think. So taking it easy in hell mode, in areas where they can appear, is essential.
D2:R