-learning how the steam acts at the stages of the steam cycle
-adjusting tip location to vary results
-probably more things
because it's a pump actuated steam pressure system, the steam will start out weak and then build up to a good flow. the key (which you seem to understand anyway) is to not try and get any decent microfoam or texturing done until the steam is at full pressure otherwise it'll just suck a ton of air into the stream and create big bubbles. this is when the steamer makes the really high pitched squeaky noise that i'm sure you've heard.
the "ch-ch-ch" sound is where decent foam is made, ideally you don't want to hear anything, just a smooth "shhhhhhhhhhhh" noise. at least that's what i find on my 820.
i judge whether it's too noisy or not by if i can talk to someone at the table about 1.5 metres away and can hear them properly :P
i also like to start out with the wand nearer to the edge of the jug to create the whirlpooling and increase the milk volume, and then bringing the wand back towards the centre of the jug after it's settled down and is sounding quiet. i find that this helps to eliminate the dead zone that whirlpools create just behind the wand that you may have noticed.
i notice it often, the milk will look great when it's spinning but just behind the wand where you don't usually look there can be a small patch of dodgy looking foam. this can sometimes wreck an otherwise good looking jug of milk. but by moving the wand into the centre it reduces the dead zone and stops the foam from gathering there.
anyway, they are some of the things that i've noticed on my 820. i've made coffees on my friends 800 and found it awkward using such a thin steam arm compared to mine but it still went ok.
above all though, practice is the key, so keep at it.