Quake 3 is absolutely remarkable in terms of multiplayer fun.
First off, the level designs are awesome, very simple to learn how to get around and straight forward while they still have a lot of substance to them. You don't need to play the levels to many times to learn where the items are and action hot spots. There are also not a lot of levels in the game, only the dozen or so of good ones compared to other games out there that have many levels but only a few that are played on. Also, if you want to create your own levels or get familiar with level design I recommend this as a good place to start. There are many tools on that internet that you can download and use to help you get started.
Next up is the weapons. These weapons are for the most part greatly balanced and different weapons are perfect for certain situations thoughout the game. Unlike the other 2 quakes in the series, the rocket launcher is not an insanely overpowered weapon, sometimes it is best to use, and sometimes the railgun, or shotgun is.
All in all practice makes perfect in this game and I am sure most people will not mind putting in the little amount of time it takes to learn the basics of the game.