The competition/challenge level in this game is extreme to say the least..
Basically Quake is about 2 things - professional 1 VS 1 gaming and a challenging single player experience.
Since Quake was the very first game to utilize the all-around-look and free movement in any 3D direction you want, one definitely should compare it more to any modern day shooter rather than with the original Doom. Quake is so much more superior than it's predecessors, that it is easy a revolutionary game in gaming perspective, just like original Wolfenstein was a revolutionary FPS game in it's own perspective.
In it's simplicity Quake is a game where you control a man in a first person view and take on a mission to clean the many dimensions and fantasy places from the many monsters using many powerful guns and icons. Can not be said more simple. Once you learn the secrets of the engine and discover that you can execute difficult jumping techniques, you understand that the game is very rewarding for those, who have truly understood and learned how good can you be in this game, that is - how fast can you run, how many monsters can you annihilate in the shortest time possible, what obstacles can you bypass, which seem impossible at first and what jumps can you perform to reach distant areas or secret places. Once you master all that, you realize that you are a better player than the rest and can do many things in Quake far better than other people - this way a competition develops. This was also the fact that Quake became the first professional gaming icon.
Different men have different reactions, different aiming skills and different ways of understanding how a game should be played. Quake is designed for people who want to take the gaming to the next level and learn conventional techniques and develop extraordinary skills, separating them from other people, playing other ordinary games. The story does not exist here, the singleplayer does not have any depth to it, but it surely is not needed anyway in this game. The sole purpose of the game is the evaluate the effort of killing and running to the max of your abilities, leaving all other things behind. Quake does that job so good, that it became the most popular game for speedruns and perfect runs of all time. Competition in this game is more than enormous - it is sick - if you can do something 2 seconds faster than the previous guy did, it is already a big achievement and a noticeable improvement. That way every corner of the game has been "washed out" so good, that people who compete at Quake must dream of it and know every level detail in mind.
For those who want to play Quake without the attitude it requires, it might be boring, because it is certainly not for everyone. Even though that might be the fact, Quake technically is still a solid piece, with the best graphics seen in a game and lots of great in-game sounds to accompany it's level details. And the levels are looking good and made in a really creative curve. Sure they are all just total fantasy, although each one tries to implicit some style - either if it looks futuristic, medieval, grotesque, cosmic or militaristic. There are even levels in Quake, where the gravity is very low and you can experience the full vastness of freedom of movement. There are many difficulty levels to choose from, and rightfully so - experienced players rightly deserve the max challenge. Choosing harder levels unlocks additional rooms, like my favorite - Vertigo, with the mentioned low gravity. Also the game has lots of secret places to explore, those contain powerups, weapons and armor. But in general the levels in Quake are too generic, not big and sometimes you will feel like playing without any continuity. In the end of each level all your statistics are registered in a score tablet, which you can compare or improve. This is the main attribute that makes Quake single player challenging.
And the multiplayer gamers have plenty of space to develop their own custom maps and mods. Quake is certainly fun to play with many guys in one server, but it really is designed for 1 VS 1 competition. That competition is so big, that newcomers to this game will certainly feel frustrated. There is much to learn in other means. Little difference in skill might result in a very contrastic results like 30/3, while if you mix up an experienced player with a casual gamer, be 100 % sure that the casual gamer will not get a single frag, and the game will simply be a total dominance by the experienced guy. This skill difference is not that noticeable in later Quake games, but it comes back in Painkiller - a true sequel to Quake in every possible way.
So there you have it - Quake is hands down the most revolutionary and challenging game of all time, although certainly not the best overall, because it is not for everyone.. It's a bit lackluster in singlepalyer as well as unbalanced in multiplayer, so the game is not superb in my eyes, but enough not to overlook.