I will share some of my experiences, some which vary from dodo, but what he said is extremely valuable
Guitars: I like to keep them as natural as possible, no compression, just trim here and there. Is obvious to roll of the bass at about 40-60, depending on how much bass you were kicking out of the amp and a low pass at 12k (the andy area, its tried and tru for me to, always keep coming back to it). PLaces for eq you should cut or boost. for extreme agressive metal, taking scoop of the 400 area (effective from 250-700, don't know what the q to that would be). Depending on the cabinet and mic and amps settings, guitars are naturally prone to have over the top 2.5k area. I take that out pretty heavy with my guitar amp to begin with and then take a peak out post recording as well. I like to boost the 6-8k area (effective from 5k-10k) followed by an immediate high rolloff at about 13. To recap, 400 scoop, 2.5k scoop 6k-8k boost with a 40 high pass and 12k low pass. Its not an exact science but that its what works well for me to get rid of those annoying peak resonances. If the tone sounds fucnky, redo it, don't record something with the hopes of fixing it with eq, get it as best as you can before you record.
Bass: Just like Dodo, make two tracks one for the lows (lowpass and compress) and highs (highpass and distort), make sure your corssover points are the same, for example if you want the slpit to happen at 200Hz poth the high pass and low pass should be set to 200, distort and compress as you wish and then buss both tracks to one bass track so you can add some powerer amp and/or cabinet modeling with some eq.
Drums: What I just found out recently, which i think is totaly awesome...first for my snare i like to roll off everything below 350 and compress until i hear a very distinct pop, 4:1 ratio, 20-30ms attack and 100-150ms release and add some reverb. I actaully like to do the same thing with the kick as the bass, separate them but instead of compressing the low end I only separate them so i can distort the highs to compensate for using raw samples and i can't mix in a trigger "click". Contrary to what was said before, I read something from andy (or maybe someone else) that was mentioning compressing the room mics and then bringing them up until you have the room sound that is desired. I usually do a highpass ont he overheads at 600 and do nothing else to the overheads other than bring the 5k+ shelf up maybe 3db for more sizzle. To the rooms, I don't pass anything, I see the rooms as the overall image of the entire kit and the reverb from the room. When i read the little bit about compressing the room mics to hell I gave it a try, what I found out was that the cymbals won't pump like expected (in the entire drum bus solo'd) what the compression does is pumps the reveb, allowing it to come through at the inital peak of the transient but cutting it off to let the decay of the closed mics and overheads to shine through. So in escence, you are compressing the room mics to controll the reverb. I came to something between 8:1 and infinity:1 and same settings I posted for the snare (20-30 attack, 100-150 release), I will bring the threashold down to the point where you can clearly hear the pumping if you solo the room mics. Wne you start bringing them up in volume to the rest of the kit, you get the huge room sound but with controlled reverb that isn't boomy or out of hand. Some will say to prevent that to high pass and remove the lows from the mics and don't compress but the way I do it, you still maintain all the punchy low end that is extremly tight and well controlled and IMO sounds way better. Like all the other insrtuments, bus all the drum tracks out to one track to you can do overall eq and add light reverb to the whole kit. DO NOT use the drum bus to do more processing.
I couldn't tell you about vocals becuase I fail at it.
Master: The best quck and easy way to do it. Use parallel compression and send both tracks out to the master bus, with this the idea is to get the compression to bring down the RMS and glue the instuments together without being obvious, bring the compressed track up until you hear some punmping and then back off until it disapears. On the master bus: Eq a shelf for the highs and lows, bring the lows from about 120 and the highs from about 8k up about 2 or 3db and follow it up with some extremely light reverb to help make the instuments sound like they are being all played in the same space. The last few touches I could recommend are to maybe use a tube or tape saturator (use it lightly, its just for adding some warmth) followed by some light limiting, just graze off the edges and nothing more. In metal the most obvious sign that you overlimited is that the kick drum will sound funny, which will be remedied by turning off the limiter.
Remember do no overprocess, that is what will prevent the pro sound. Use everything lightly, including eq and reverb, try to do as minimal as possiblebeucase the most natural sound is what results as the best, most "pro" sound.