As Snout said, it depends somewhat on your playstyle, however heres an overview:
For PvE, you'll be wanting to keep 2 sets of gear, one thats completely toughness/armour based, and includes a shield and 1 handed axe. The 2nd thats all strength, and includes a 2handed axe.
The reason for this is simple, while your questing, you want to kill the mobs as quickly as possible, and str/2hander is defineatly the way to go there. Stage 1 PQ mobs also fall into this catagory.
For boss fights, lair fights, dungeon fights, PQ hero fights etc, you'll be wanting your toughness gear and a shield.
Since PvE really is easy mode, you really don't need to focus on the other stats that much unless your only ever going to solo.
For PvP,it changes a bit as you rank up.
Toughness is still your primary stat, however it really starts to become based on your playstyle/mastery path.
You can forget Ballistics and Intelligence, they offer you nothing.
Regardless of your path/style, you'll need as much toughness/wounds/armour as your gear will allow. You are after all a tank. Armour is pretty obvious, it directly reduces the amount of melee damage you take. Toughness helps mitigate all incoming damage, and more wounds = more time for your healers to react to the enemy turning thier attentions to you. For tier 1, 2 and probably the start of 3 this is pretty much your bread and butter.
After that you need to start thinking about what your setup to do.
Strength helps increase your damage, but lets face it, who cares? Your role unless you have alot of tanks is a CC one, not a DD one. The reason you want strength is because it helps reduce your opponents chance to block or parry and attack. Nothing is more annoying than an enemy blocking your knockback. Try and get this stat as high as possible without sacrificing your toughness and wounds.
Weapon Skill helps you parry attacks and penetrate your opponents armour, however while the parry is nice, if your group is relying on you to DPS the bad guys down then your probably in trouble.
Inititive may seem like a wasted stat, but the higher it is, the lower your opponents chance to critically hit you is. Less crits = you stay alive longer = your group stays up longer. It also serves the dual purpose of helping you detect those pesky stealthed witch elves that are lurking near your healers faster.
Willpower is primarily for healers, however its also your magic defense stat. A higher willpower helps increase the chance you will disrupt an incoming enemy spell. Disrupt btw means you resist it, and take no damage. This becomes important because you can take a fair beating from other melee, but sorcs and shamans will hurt you bad the higher rank you get.
Which leads me to your defensive stats, i.e Resists. These may seem worthless, but are actually very important for the same reason willpower is, namely your weakness is magic damage. Higher resists mean you take less damage from the sorcs and shamans, which again means you live longer. How you balance the 3 resists is up to you, but Sorcs do more corporeal damage than anything else, and Shamans do mostly elemental damage.
Don't forget as you rank up that you get several tools to help you there, including a tactic and an ability that both boost your corporeal resistance.
You'll also need to start thinking about your weapons. If you put enough points into the Vengeance Mastery Path, then you'll pick up at least Cave In, which is our best single target knockdown and also one of our better damage moves. It does require you to be using a great weapon though, so start looking out for one with lots of wounds, toughness and a bit of strength if your lucky. Grudge born fury is also a greataxe required skill, is one of our better damage skills for when you do need to hurt someone, and also has possibly the coolest animation of any of our attacks.
Basically you need to find your mix based on what you face most. Since PvP is fluid and your likely to face all types, ignore the lesser stats at your own peril.
I'm finding that as I edge closer to 40, my toughness hasn't increased much for the last few ranks, but my wounds and resistances have jumped up alot, and I'm starting to consider gear for its all roundedness, rather than just being the toughness junkie I was for the earlier tiers.
Hope this helped a bit.
Morgrim - Ironclaw Server
Rank 40, Renown Rank 31
Immortal Guard Guild
Loaar - Retired
Galka, Pheonix, Serene LS
RDM 75 / PUP 75 / NIN 75