It's an over exaggerated, basic, and silly good vs evil story
Basic isn't bad at all. Suikeoden 2 (one of the best RPGs of all time) has a basic story. Also what is exaggerated? Do you mean the voice acting that become very iconic to the game?
Good has very little to do with the actual story of the first game after the fight between the angels and demons which is a manual only story segment. So I am not sure what you are talking about since it had nothing to do with D1. The story type is "Good Vs. Evil", get your steriotypes correct. It is "Knight vs the devil" which is about the corruption of the hero.
As well it wasn't all that silly except for a few things here and there that were meant to be silly (Like the Imp quest). It was also a very vivid story and the execution brought it to life.
In otherwords D1s story felt like a story and not a series of excuses. Even the monster bios were deeply interesting and MUCH more so then the next games. In fact the D1 story was soo tight they had to rewrite several large portions of the story in d2 and d3 and manages to create HUUUGE gaping plotholes.
Ohh goodness the plotholes... D3 you are clearly the master at creating plotholes.
D2 started out as a pile of shit compared to what it is now today
D2 added two classes, charms, rune, Magic Gems, and a few extra items as well as a final act of which only 1/4th was any good as well as a new final boss inferior to the old one. The Expansion did a bit of refinement but lets not stretch the truth here. The expansion did not reinvent the game.
As well Vherid one of the largest reasons why D3 is weak right now is they had to reinvent the game quite a few times and it shows. Such as in the rune system.