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Welcome to warrior ^^. In the thread above yours i posted a response to changing talents, so i'll try not to re-iterate.
This is only IMO, based on leveling to 35 as a tauren warrior. With that in mind I will go through the usefullness of positions as i've noticed them.

Arms ~ i have no qualms with this stance, it's your starting stance and a nice evenly spread defense and attack position.
Charge-requires no rage, great lead off/distance closing/stun attack. When you get this skill don't be surprised to find that you almost always lead off with it in best scenerio situations. the minor rage accounted by it, helps out.

Herioc strike ~ I have mixed feelings about this skill, always seems to me to delay the attack slightly, the added damage helps. It's really up to you while trying this skill to develope an opionon. Personally I prefer not to use it and focus on buff and debuff, as well as saving some rage in reserve for the unexpected complications that sometimes arise in battle.

Counterattack ~ is GOD when you first recieve it, when in a dire situation vs. many opponents gather them in front of you, and click this little nifty option. Preferably weilding a nice two handed weapon. If a rogue is new to fighting warriors and encounters this, it can be devestating. It's only usuable every hour, so make sure the situation is not completly doomed, best not to waste this. Fighting a quick attacking melee enemy, anything that dual wields, or uses one handed weapontry this skill immediatly used will usually turn the tide, or used immediatly, crush your oppenent human or boss. You get this skill earlish on in your carreer, you will always find use for it.

Fury~My heart felt favorite. In the thread before yours i spoke highly of it, and should not because of this write that much here.

Slam- basically the same as herioc strike, I will experiment later with putting points into it to see if it's worthwhile, however it works just like herioc strike, minus the huge fact that it counts down like a spell that can be interupted. As you have played mage I imagine you well understand that spell interuption while 1 foot in front of the enemy is going to be a problem, so look not far into this skill.

Intercept~is just like charge but better and worse, the stun is much more impressive on this skill, however you have to have rage in order to use it. The need for rage, renders it useless, unless you have rage built up from a previous battle. However if you have it, use it, great stun.

Most of the usefullness you get out of Fury is the passive bonus's like having close to a 12 percent constant chance to hit criticals, enrage which pumped up allowing 15% health regeneration during battle, and 40% increase to damage activated when critically hit, and flurry increase to weapon attack speed when dealing critical hit, along with all of your buff/debuff shouts. Not under that list are.

~Intimidating shout. Fantastic when mobs rush you, or pvping in a crowd of enemies, AOE fear offect for 6 secs to everything but the poor fellow your fighthing. Killed a druid once that was healing the paladin I was engaging, sent the paladin away, left the druid quite screwed.

~Challenging shout. Great, summons all enemies near you to focus attack on you, fanatastic for instances gone awry. In PVP useless of course, and there is a 10 minute recast time on it.

If you wish to know how a Fury/Berserker stance warrior fairs against Fear using Warlocks and what not, read the thread ahead of yours, I covered it there. But we stand very well against fear with two of our skills, even if we are not using an undead.

Defensive stance. Im not sure how you will like this one, personally i prefered Arms and berserker stance over it, but that's my opinon. Beyond sundar armor taunt and disarm, this skill mainly requires a shield, im not so keen on that. Personally i've noted using Berserker/Fury that I hold better aggro in groups by simply criticalling, and pissing the mob off more than anyone else in the group, but this is going to require you to have strictly updated weapons and armor for the aggro, which if you want to be a sucessful warrior i would highly advise doing anyway.

Remember also, if you are defensive in many mob situations, where everyone seems to have someone on them, it will take you more time to kill your opponent and help them out if your weilding a shield and sword and taking your time, better to maul through your enemy and move on to theirs.

If you play warrior well, you'll be surprised at the dps. we actually can dish out, and the advantages of our debuffs and buffs. If you have other questions, as im sure I was not able to cover, let me know.

Good luck.

My personal favorite abilities, so far, in the order I typically use them in battle:

Rend: WoW is the only MMORPG I have ever played that gives warriors a DOT! I love Blizzard for this, and I used talent points on the whole Rend-ish line in Arms. I find that Rend seems to hit a high percentage of the time, even against drastically higher level opponents (had no trouble landing it on Stitches when I was 29). With my current level and talent bonuses, Rend does 15 damage every 3 seconds. That's 5 DPS, and when my shiny new sword is 23 DPS, I am lovin' another 5!

Sunder Armor: Like Rend, it can be used in multiple stances, so any warrior can probably use it. Even at rank 1 it is very useful, but starting at rank 2, it becomes ridiculously powerful. Since it can be stacked 5 times, at rank 2 it is possible to drop an opponent's armor rating by 900. I can't see any way to knock that!

Execute: Easily one of the most maniacal-laughter-inducing abilities a warrior will get, but it sees little use unless you focus on battle stance (solo'ing, usually). The ability to channel excess rage points into one big shot (usually going over 500 for me at level 30) as a finishing move is really a beautiful thing, especially when dual-wielding. Dual-wielders will really love the fact that the damage base is fixed, so even a dagger in the main hand can lay down the rank 2 max of 800 damage... heh.

Demoralizing Shout: A nice, though not overly powerful area-effect debuff. I nevertheless find it a cheap and frequently usable way to draw adds to me. Aids in solo'ing, as well, by drastically increasing survivability against multiple targets.

I tried to keep this as general as possible, showcasing abilities usable in multiple stances. I abuse the Anger Management line of talents to switch stances frequently in battle, so these abilities are provide a sort of continuity between stances. Since they are always available, I don't have to change stances quite as often.