Sets the tone.
The difficulty increases gradually, there are points where it seems impossible to continue, in that case you can always start a new game in order to gain some more level and skills.
The worst part about the game-play is that your character can only walk, which is quite annoying if you're inpatient.
Characters: You have three characters: The warrior, the rogue and the sorcerer (all of which make a cameo on Diablo II). Unlike Diablo II your character can learn any spell (from books) and use any weapon (as long as he/she fulfills the requirements) so the only real differences between them are how their stats begin and how the weapons and armor look on them.
The people on the town are there to help you with your Quests. There's a healer, a blacksmith, a witch and a kid who sells rare items (most of them very expensive and not that good) but the son of a gun will charge you just to take a look, so unless you have lots of gold is not worth it. And Deckard Cain, the last living member of the Horadrim (the guys that imprisoned Diablo). The rest of the characters (the drunk and the lady from the tabern) are there mostly to help develop the story.
Plot: The story is very simple, the town of Tristam has been invaded by demons, their leaders have been corrupted and most of the people that lived there are either dead or left the town. Once you go deeper into the labyrinth you learn more and more about the truth behind everything. Playing the game and discovering the plot is the best part of the game, so I won't spoil you more than I already did (if you already played you know what happened so there's no point in repeating it).
Graphics: For a game released 10 years ago, the graphics are still acceptable today, they really help setting the environment. This is where Diablo II failed a bit, while Diablo succeeded to create an environment of darkness and decay that could actually scared you.
Sound: The music is a classic, not many games have a soundtrack as good as the Diablo series. However the character gets a little repetitive with his "not enough mana" and "I can't cast that here" but that's part of what makes Diablo what it was, one of the greatest games of all times.