There are dozens of threads here and on the WotC site, where confusion and disagreement about stealth are quite apparent.
Disagreements by people that claim things like "hidden from view" is different from "hidden" or "can't be seen" and make assumptions without actually reading the rules.
1. A character needs concealment or cover to make a stealth check. Allies provide cover against ranged attacks. Can I stand behind an ally and use stealth to hide from my enemies and make sneak attacks against them?
Rules as written, yes you could do this with ranged attacks (allies do not provide cover from melee attacks.)
Combat Advantage merely means your target can't actively dodge your attack because your target can't see you as you aim.
This is easy to counter. On their turn, your target can simply move to change their line of sight and get a clear attack on you.
I do admit I look forward to a FAQ clarifying the issue. I wouldn't begrudge a DM who didn't play it by the book. But there it is.
2. I hide behind a tree. As long as I am in that single square, I have cover and concealment. If I move out, I am immediately spotted. Can I pick my nose using stealth as a free action, declare that my enemies are now "unaware" of me, and use sneak attack? Can I do this repeatedly, round after round, standing in the same spot, sneakily picking my nose?
The Free Action Complaint:
"Part of Whatever action you are trying to perform stealthily"
A free action typically doesn't hide you - it just means your free action was performed stealthily. Ie, nobody saw or heard you pick your nose, whisper to your friend, or drop an item (typical free actions.)
Not every stealth action makes you unseen. If you merely want to do something quietly (say, pick a lock) you use the stealth skill. "Part of whatever action you are performing stealthily" does not mean you are always unseen with every action. Sometimes it only means unheard, sometimes it only means avoid notice.
But for your Pillar Example:
Behind a tree: you have cover. Not concealment. Not the same thing.
If you successfully steatlh, hiding behind the tree with a move or minor action: then you are hidden from view, unseen, able to attack with combat advantage and requiring that attacks against you are at -5 (can't be seen is the definition of Total concealment.)
When you attack: you lose concealment.
Then you must make an action to hide again. Note what I said about free actions above.
You must make an action to move yourself back behind the pillar, to take advantage of the cover to stealth. Sounds like a move or a minor action.
3. Is the enemy's perception check active or passive? When and why? If the enemies make active perception checks, we have maybe 20 goblins all trying to spot me while I pick my nose behind the tree. This happens every round. And maybe my allies are making stealth checks too. Isn't this a bit much?
Here you are deliberately being obtuse or not reading the rules.
Yes, it does say on page 188:
Opposed Check: Stealth vs. Perception (see the
table for modifiers to your check). If there are multiple
observers, your Stealth check is opposed by
each observer’s Perception check.
But if you were so bold as to look up what Perception says on page 186:
Perception: No action required—either you notice
something or you don’t. Your DM usually uses your
passive Perception check result.
As poster Innuit says, think of your passive Perception as a defense against stealth.
You make one stealth roll. You compare it against the highest passive perception skill of the enemies that might see you. That one enemy, if intelligent, can use a free action to alert the rest.
One roll.
One target number.
No. Big. Deal.
Now - in their turn, your enemies can make active checks. I admit there is an apparent inconsistency here.
On page 186:
Perception: No action required—either you notice
something or you don’t. Your DM usually uses your
passive Perception check result. If you want to use the
skill actively, you need to take a standard action or
spend 1 minute listening or searching, depending on
the task.
Which would seem to tell you that you need to take a standard action for an active check.
However, as the book says, Specific overrules general.
We have a more specific rule:
Page 281, Targeting What You Can't See:
Make a Perception Check: On your turn, you can make
an active Perception check as a minor action...
So it is only a minor action for an active perception check against an unseen (ie, hidden from view, stealthed) target.
4. Making an attack means my enemies are no longer unaware of me. What do I need to do, stealthily, to make them unaware of me again. Do I need to change location, do any action at all with a successful stealth check, or just wait for my next turn?
Please cite the magic "unaware", change location, etc., as a requirement to stealth.
Yes, you need to do an action. "Part of whatever action you are performing stealthily". No you can't simply attack stealthily: attacks break stealth.
See above for free action.
If your attack was your last action in your turn, you need to wait for the next round to hide.
5. What does "unaware of me" actually mean? That my enemies do not know that I exist? That they do not know my general whereabouts? That they do not know my exact location? That they do not know the details of what I am doing this moment? The answer to that question has a big impact on how useful stealth is to establish combat advantage by the "enemies unaware of you" criteria.
Please reread combat advantage. It specifically sites page 188 (stealth) as one of the ways to gain CA.
Stealth has many uses besides hiding. It is moving silently. It is (aside from specific uses of sleight of hand) performing actions without being noticed.
It could even be mixing in a crowd without the guards looking for you noticing you.
It is not a Jedi Mind Trick. The people you are hiding from don't forget that you are out there somewhere. If you are hiding behind a piece of solitary cover (like a tree), they likely will know exactly where you are. They just can see you.
4th edition skills are broad, and don't do one simple thing (ie, Move Silent).
Arcana is detecting magic, spell identification, ritual use, magical lore, etc. Stealth is also many things.
6. If you pass a stealth check against one enemy, but fail against his buddy, do you assume the buddy lets him know what's up and ruins your combat advantage?
It is a free action to communicate. If you're facing intelligent enemies, why wouldn't they?
The official replies haven't been entirely consistent with each other, or with the RAW. We get the impression that rogues are supposed to be using stealth -> enemies unaware -> combat advantage -> sneak attack almost all the time, but that's a chain with a number of links to it, and a common-sense approach would often result in at least one of those links breaking fairly regularly.
Customer service is not official. They can be helpful at times, but their answers don't have any kind of quality assurance process. The errata and the FAQ are official. I suggest you
check it out.
There's also a confusion about stealth making the enemy "unaware of you" and the rules about unseen adversaries. Unaware = unseen = invisible?
Again, check the FAQ.
But from the PHB:
Stealth, Success: You avoid notice, unheard and hidden from
view.
You want to pick your nose in the throne room without upsetting the King? That would be stealth, but it wouldn't make you unseen.
You want to hide behind a pillar so you can jump out and attack? That would be stealth, but hiding behind the pillar would make you unseen unless your enemy had x-ray vision or tremor sense.
In a rules set where almost every condition is very clearly and unambiguously defined, both in terms of its causes and its consequences, we are having to guess our way through the condition "hidden"/"enemy unaware of you", both in terms of how you get that condition and in terms of what you can actually do when you have it, without losing it.
The people that can't make the leap between "hidden from view" and "can't be seen" need professional help.