What stat do I prioritize as a Guardian?
Hit (7.5%) > Expertise (15%) > Crit > Haste >>> Mastery > Dodge.
Crit > Hit/Expertise (until comfortable) > Haste >>> Mastery > Dodge.
Essentially get comfortable with your rage generation so you can keep up your active mitigation abilities. Crit will give you more rage if the RNG gods love you. Hit/expertise caps give you a steady generation, allowing you to comfortably keep up stuff and allows you to predict when you can use certain abilities, moving straight into:
How often am I supposed to use Maul?
Anytime in 5man dungeons, when you are not tanking in raids, when you close to 100 rage and Savage Defense is up, or when the damage is not dodgeable and you do not need an immediate heal (>80% health or so).
Is it more important to use SD or FR? SD against melee/physical and SR against magic?
SD when the damage is dodgeable
. Bleeds are physical damage and cannot be dodged, so you use FR here (ie, Stone Guard in MSV). Other wise, yes. SD on melee damage and FR on magic damage. Also use FR when you are in immediate danger of dying to bring yourself back up.
Any general tips for holding aggro
Attaining Hit and Expertise caps will help this immensely. Other wise utilizing Mangle every single time it is available, and Swipe > Thrash > Swipe (although I'm bad and have a macro for Thrash > Swipe > Swipe) will do for AoE. Remember Lacerate damage is front loaded (big hit when it connects, and a small bleed after).
usage of spells would be great as well
You use Mangle to generate rage. Lacerate, Thrash and Faerie Fire will reset the cool down on Mangle. Your melee crits will also generate rage. You use this rage to keep up Savage Defense to deal with dodgeable damage, or to heal up with Frenzied Regeneration.
Thrash applies Weakened Blows and you want to keep it up to as close to 100% uptime while not clipping it. ie, Mangle > Thrash > Faerie Fire > Lacerate
Oh, and is there anything Guardians do better than other Tanks?
We take large hits to the face better than other tanks. We also have the option to take Heart of the Wild for 45 seconds of very high DPS or very powerful heals (particularly with Tranquility). We are also very mobile and have the tools to kite fairly well (speed boosts, Vortex/Typhoon, etc).