After having to reject several GMing applications after their initial sample game didn't meet expectations, I have compiled a list of critical guidelines and examples of what you should and should not do as a HoA GM.
Requirement 1 - Have a fair grasp of the rules: All GMs should have a fair grasp of the rules. This does not mean that you should know every rule ever, but it does mean that you should have a fairly solid grasp on the core mechanics, how to set DCs for skills, abilities, and so forth. Knowing how to reference something when in doubt is also a good talent. If you do not know, feel free to ask your players if they know. If you need to come up with a rule that doesn't exist, or your group doesn't know either and you need a judgment call, try to base your call off the standards for the system.
Example 1 (Good): One of your players decides he wants to throw a bag of chalk into an orc's eyes to blind him. This isn't covered in the rules, but you look at the situation and decide that he should probably throw it like a splash weapon (a touch attack). Then you decide to give the orc a Fortitude save to negate the effect. Because you are unsure of the save DC, you decide that you will set the save DC at 10 + 1/2 the thrower's base attack + thrower's Dexterity modifier because the 10 + 1/2 level + ability modifier is a common formula, and you decide to make it based off base attack and Dexterity because it represents him actively trying to throw it at his eyes.
Example 2 (Bad): The same situation occurs, but instead of looking at pre-established rules, you just say it's an attack roll and if it hits the orc is blind. Or you decide to roll % dice to decide if it works or if the player can do it. Or you have the orc dodge or ignore the attack because his Constitution is above average and he would just "tough through it".
Requirement 2 - Don't Screw with Players: As noted all GMs should have a fair grasp of the rules since it helps them make judgment calls. However, GMs should not strive to invent new problems for the player characters. There are plenty of challenges in the game and when it is run by the book it is actually a very difficult game. The game is at its foundation a fantasy and the rules are an abstraction. You should not require a check for every little thing, nor should you invent checks or drawbacks that do not exist as part of the game. While it is alright to apply established adjustments to DCs as appropriate for the game, you should not begin making up rules to create additional problems for the PCs.
Example 1 (Good): One of your players decides to attempt to climb a rocky cliffside. This particular cliff is covered in a slick and fairly slimy moss. You decide that is probably not helpful to climbing it at all, so you assign a +2 circumstance penalty to the DC to climb this rock because it is slick and wet. Later, a gunslinger is caught in a battle in the middle of a downpour of rain. You decide that the downpour makes seeing clearly difficult, and everyone in the rainstorm has concealment (20%) while in the storm, and apply this circumstancial complication to both the PCs and NPCs. Later still, a player with a low charisma score attempts to talk an NPC into letting them cross a bridge he is guarding. You decide that the NPC does not know them but he could get in a little bit of trouble for letting them cross without permission, so you set the NPC's attitude at Indifferent (DC 15) with a +10 to the DC (for giving aid that could result in punishment) for DC 25. The PC's low Charisma will hurt his chances, but with enough ranks or feats he might be able to do it.
Example 2 (Bad): As above, except that you decide that the player cannot take 10 arbitrarily, and that if he misses the DC at all (not just by more than 5 as per the normal climb rules) he automatically falls off the cliff. Later, during the fight in the rain, you decide arbitrarily that the gunslinger can't use his weapons in the rain because you think guns shouldn't work well in the rain, so you say the gun locks up or jams on a roll of 1, or disallow its use entirely. Later still, you decide to set the attitude of the NPC bridge guard based on the Charisma score (a metagame number) of the interacting PC. Since he has a low Charisma, you arbitrarily make the NPC unfriendly in addition to applying the usual modifiers (resuling in a DC 30 Diplomacy check).
Requirement 3 - Building Encounters and Awarding Treasure: Rules aren't just for players. Us GMs have to follow the rules too. Part of being a good GM is about building trust and confidence with your players. By following the rules and guidelines of the game, players will both respect your authority, and will accept misfortune more gracefully as it is then just a part of the game as opposed to a result of bad GMing. A major part of being a GM is also understanding how to build encounters and award treasure. You don't want to make encounters that are too hard, too easy, or give too much or too little treasure. It is important to make sure that you are not only familiar with the rules for building encounters, but that you follow them at least closely (if not perfectly).
Likewise, knowing how to run an encounter properly is a big deal. A GM should strive to make the encounter appropriately difficult. That means being familiar with the monsters, traps, and situations you plan to use. If you plan to have en encounter in the dark, refresh yourself on the rules for lighting, blindness, darkvision, low-light vision, and so forth. If you plan to use a monster with various abilities, make sure they use them (don't forget things like Perception checks, Scent, Spells, etc).
Example 1 (Good): You are building a standard encounter for a 4th level party (CR 4). Looking at the GM rules, you find that such an encounter should be worth 1,200 XP worth of enemies. Your adventure involves a lot of orcs (worth 135 XP each) so you place about 8-9 in the encounter. During the fight, one of the party members is struck by a particularly devastating blow (a critical hit with a greataxe) and is killed. The player, seeing that the rules were followed closely, realizes this was merely a natural result and the game continues. Later, the players decide they are going to sneak up on a group of wolves. You apply penalties for distance to the wolves' Perception checks. You decide the wolves are not actively looking for the party so you assume they are taking 10 on their perception checks. You also note that they have scent so if the party members get to close, the wolves will know something is there, even if they're not sure what it is (which denies a surprise round, might make the wolves flee, or actively look for the source of the smell). Later you are running a higher level game and notice that the demon that you are using has the ability to teleport around as he likes. You have him use this ability to transport himself to weaker members of the party, or to quickly get to hard to reach places to cast spells on the party. The group is harried by the demon who is making use of all his abilities properly.
Example 2 (Bad): As above, except that you decide to drop 50 orcs on the group without any rhyme or reason for the encounter being so huge, with no real option to avoid the encounter or flee. One (or more) of the PCs die, and you are soley responsible. Later (after everyone re-rolls characters from your god-aweful encounter before), the party is hunting wolves. You allow the party or the wolves to take 20 on their checks (which means you likewise are screwing up the rules). During the encounter, you never account for either the distance between the party and the wolves, or for the wolves scent ability. You let the party ambush them by sneaking right up onto them and slaughter them without much of a fight, and award full XP/treasure. Later, you are using the demon, and despite having all these really amazing spell-like abilities to help him be mobile and harass the party, you have him charge headfirst into the party's fighter and commense with getting his meat grinded (and not in the good way) before passing out XP for "defeating" this terrible demon.