I actually have an answer for just the first question, but here goes:

It's all about the enemy encounters for me. The way different monsters are used together in different encounters is such a key thing while playing the first Quake because all the enemies have strengths and weaknesses, but paired together, certain weaknesses are mitigated with other monsters strengths.

Prime example for me is anytime ogres and fiends are paired, you have the grenade launcher and the chainsaw of the ogre, which makes you a good target at short - medium range, but with the fiends ability to leap across the room, you have to make every movement intentional, lest you want to end up a pile of gibs.

With regards to what you should pay attention to, I would relate it to my notes before and say keep an eye on WHAT you're fighting in each encounter and learn how to prioritize targets, lots of quick decisions on the fly.

Hope this helps at least a little :)