One major issue in the single vs. multiplayer section!
You forgot to mention that multiplayer is greatly harder than single player. There is a significant XP limit per kill in multiplayer, which makes it A LOT harder to level up. Also, all the monsters have 2xhitpoints in contrast of single player which makes MP yet again harder - it takes twice the time to kill a monster.
You can play Nightmare and Hell modes in SP - the single player just picks up whatever difficulty mode is last used in multiplayer. So create a nightmare multiplayer game, quit, and create a single player game - whoah, you've got a nightmare SP game.
On a more minor note: The default key for highlighting was ALT in D2, imho. SHIFT, on the other hand, is used to keep the character from moving, and it is really useful in Diablo 1 - Warrior can block doors and Rogues use it almost always when firing bows.
You could also stress the point that even if Sorcerer is the most powerful class in the end-game, it is very frustrating to start one. If I were to read your guide, I would assume Sorcerer is the way to go, die couple of times kicking an exploding barrel and assume that Diablo 1 is uber-hard HC game
Oh, and unlike D2, some of the best items of the game are just simply blue magic items! So people won't rely on uniques too much.