As said before, caring about the group's need is basically what most people would want of you. My take on the Wa/Mo :
- you're a target : you have the best armor (make sure you do), monsters should be munching on you, and not on the juicies in the group. Make sure you're first to aggro, and keep it. Don't hesitate to switch targets if a critter ignores you and goes for the back line : engaged monsters will probably keep on hitting you while you move, and you should attract the straggler's attention.
- you're a target : you're that little flag poking from the hole in the middle of the green. Nukers should find tight-packed critters groups near you. Herd them. Use the terrain to block them. Switch targets and move a few paces to make those 2 groups a big mob. It won't work everytime, but try.
- you're a shield : help the group when in trouble. Mesmer getting clubbered by a troll ? You got there and hit it ! It will probably choose you as the next target, especially if it's previous victim retreats.
- you are not a killer : well, you are, but not so good at it. Don't expect to be dropping mobs by yourself. No sense in taking offensive skills exclusively, you're not soloing. Watch Yourself is a cheap adrenal skill, but the +20AL will make a world a difference for softies under fire.
- you're a monk too : duh ! You'll never be a good monk, but you can help. I always have some kind of res ready. I usually pack a Word of Healing too : it's an 'emergency' heal (cast time is so-so), so you'll always get the <50% health bonus, and it's cheap (5 energy). Can't use it on yourself ; that means you'll have to rely on the team to keep you alive, then. I like Healing Seed too (no self-cast, pity), efficient when a soft-target cluster gets into trouble.
As for Mending, I never leave without it. At 10 Healing Prayer, it's a +3 regen ; combined with 80+10AL+shield+absorption+whatever, it goes a long way against melee-type opponents. Spares that much healing and energy which can be put to better use. It leaves you with 1 energy regen ? You're a warrior, energy is scares anyway. Switch to a focus when you absolutely need to cast (and that means preventing a team-mate from dying, mostly). Plus you can even _cast it on other_. True, I tested it, it works on teamies too ! Great for counter-balancing disease or poison when the whole party turns sickly green and healers can't keep up.
On a note to other classes : it's normal for the warrior to run ahead. How else do you expect monsters to preferentially target the tank ? Paint the armor red ? So please, stop trying to keep up, especially when closing in on mobs. We need room to herd and divert them, it's not as though a single warrior will kill all targets before you get a chance to bring out the fireworks ...
On the other hand, 2 days ago, a fellow member in a PUG told me I was making the mission easy (he apparently had tried with a few other groups before) : I guess it's as good as it gets

That was real nice to hear (well, read, but you get the idea).
That's it for now. Once again just my humble opinion. I would like to know what groups expect from there favorite meat shield, as much as the OP does...