It's not a Doom-killer, but its QUAKE
At first of course what we saw in Quake was the impressive graphics - a fully 3D world with dynamic lighting and such. Quake's graphics was a major step in technology of computer gaming, probably as large as Doom's step in the history of FPS games.
The sound was also impressive - the weapons sounded so powerful that it almost hurt your ears when you were listening to that sound too much, the monsters also sounded creepy, mighty and hostile, music was also great but it didn't fit the atmosphere exactly like it was supposed to however, but it was again a good entrance of something like "very atmospheric and creepy sh*t in the background, which is not MIDI".
And of course the most important thing in the game - the gameplay. Quake introduced a very great single player campaign - atmospheric, thanks to great graphical technology, decent audio, but mostly it was the gameplay itself - as already told the monsters sounded very powerful as well as the weapons - some of these were very original and to this day have influenced lots of game developers. Some weapons, like The Nailgun and the Lightning Gun, are legends in the making, while some monsters like Ogres, Shamblers will lie in our memories forever as one of the most original, creepy and coolest monsters we saw in a video game.
But there was one thing which differed Quake from any other game of that time - online multiplayer. For the first time, we were able to compete in deathmatch with not only our friends our buddies by job, but with actual strangers from around the world, long story short, it was a great experience.
It's hard to say that there is some flaws in the game, but there could be a few. The most known flaw in a lot of modern first person shooters is the repetitive level design - which happened in Quake a lot actually, but we can't help it since this game was originally supposed to be a RPG with medieval themes. Then again, the developers added something modern to this game too, like the first levels of each episodes, which where more like futuristic military bases, and some enemies were also modern-like.
The Bottom Line
Quake is a very original game, which lots of developers tried to "kill" with their so called "Quake killers" in the late 90's. So this already tells that this is a very good game. A game worth buying even nowadays, the main reason is the multiplayer.