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Living Target: You are best at drawing your enemy's attention away from allies.
Depending how difficult it is to keep aggro and how many enemies a team may be fighting at a time (a few like CO or many like CoX) this could be the go to mastery for team play. Likely not useful for solo play.

We have received confirmation that anything that mentions teams or allies provides no benefits for solo play, so that is a safe assumption. While this is likely a straight aggro boost, I would like to think that the wording leaves it open to such things as a -Accuracy debuff.

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Bruiser: Your attacks help debilitate foes over time.
If debilitate means "lowers the damage of", I like this one a lot. Good for grouping and good for soloing. If debilitate means "lowers the resistance of", its still good for both but maybe not as good for tanking imho.

Agreed. Unless the stalwart can deliver a fair amount of damage (esp. AoE damage, depending on whether or not the mastery affects multiple targets), a resistance debuff would have to be quite significant to provide much benefit. Since debilitate could be read to mean debuff, that little blurb is quite open to interpretation. "Help debuff" may even indicate that it provides bonuses to debuffs, although this would presume that all stalwart sets have debuffs.

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Battle Leader: When your allies focus on you, their attention improves your ability to defend them.
We already have a clarification on this one, awesome! When allies have you targeted (either for buffing or to attack your target), they take less damage and you hold aggro better. IMHO this may be the best tanking mastery, but not useful solo.

Definitely appears to be geared toward single target fights rather than the faster pace of taking out the trash. Definitely counts on team work, at least to the extent of using a 'my target's target' or 'assist target' function. A solid team could potentially simply run on 'assist target' mode and rely on the tank to function as fire control.

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Ruggedness: You can shrug off more damage.
I could see this one being the standard while leveling, both in groups and solo. If our damage is lower (like tank damage often is in other games), better survivability becomes more valuable. Alternately, If late game tanking is all about surviving big hits, I could also see this being an important end game mastery.

This does look like a go-to choice. Unfortunately such bonuses are usually minimally effective at low levels ("Woo, I get a 5% bonus to my 15% resistance.") and only come into their own at middling to high levels, especially if both the bonus and the stats being boosted become larger.

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Protector: Your team strengthens your resolve.
I asked in the mastery discussion thread what this means. If resolve means "increased resistance to crowd control effects" then this may be a situational, but powerful, choice for grouping but with no solo use.

I am inclined to think that this has little or nothing to do with CC, especially since it is a team oriented mastery. First, as Titan indicated, it would make the mastery very situational. Second, it would limit or even eliminate the usefulness of any powers that provide CC resistance or immunity. Could potentially dovetail with Battle Leader, providing the stalwart with great defense and/or resistance, making them the ultimate "come with me if you want to live" machine.