What should I do when the Boss is <20%? Keep up the rotation? Spam Execute at 30 Rage?
Don't use a none SD execute. You'll drain all your rage, and have to wait for a white hit to get enough rage to use anything else. Arms is all about using ever GC to do something.
I use Quartz to show my swing timer, but how exactly should I take that into account?
Your weapon swing timing no longer matters. It used to matter for timing the use of slam, but slam no long resets your swing, so it does nothing now.
In which situations or under which condition should I use Heroic Strike?
I usually use HS when I'm at 90+ rage. The fact that you do not get any rage from a heroic strike means you have to wait until your next attack to get that rage. You want to have enough to MS, possible SD, OP, and slam. So that is up to +80 rage you can use in the span after a HS with only about one normal attack. It is better to to waste rage by hitting the 100 cap then running out of rage for abilities. At the same time don't just sit at 100 rage and never HS.
I've noticed while looking through some WWS logs that many high dps warriors use Whirlwind, is it really worth changing the stance, thus losing rage, to do so?
That is Bladestorm. It shows up in logs as WW. Arms warriors don't actually switch to berserker to WW because it is a waste of too many GC and rage.
Is using Cleave as Arms any good? Same goes for using Recklessness.
You can replace the instances of HS with Cleave if you will hit two mobs. Don't spam it in AOE situations though or you will have no rage for other ablities. I generally don't use Recklessness because I haven't figure out if it's really worth it. That would require more input from others.
Should I drop Imp Execute or is it worth having?
I personally dropped Imp Execute because the warrior DPS spreadsheet showed that it's DPS increase was minimal for all the junk talents it required. I only run 25 mans, so I'm never need for the shout buffs. If you do need the shout talents then it may be worth having.
Hitcap is 263 and any hit beyond that is useless right? And the Exp Cap is 26 if i read that right? Is that having Weapon Mastery 2/2 or do I need that on top?
Hit cap is 263 for 8% (230 if alliance and you have a Draenei). Exp cap is 26 with no talents, 18 with 2/2 Weapon Mastery. Anything over either cap as arms is wasted.
Last thing I am curious about is, I noticed that - compared to other Arms warriors in my raid - I generate really low rage, although I have Anger Management + Unbridled Wrath 5/5 and I got no idea why. I read the thread about rage generation, but I'm either too dumb or unexperienced in WoW or both to draw any conclusions as of what to change from that.
You should not need AM or UW for rage as arms. As long as you are not spamming HS you should have more than enough rage in a raid situation. Without actually looking at a log it's hard to guess what any other problems could be.