[RPG] a Killswitch

The bonus is applied to every damage roll the Warlord's allies do.

In order to comprehend what a damage roll really is, read Rules Comendium, page 222. To short it up, any expression of dice (or [W]) that you can find in a power, feat, monster stat block, etc. The following are examples of damage rolls.

• 2d8 + Wisdom modifier damage
• 1[W] + Strength modifier damage
• 3d10+4 fire damage

Damage roll modifiers can be applied to such expressions. If a power, class feature or else allows you to perform multiple attacks (like the Ranger's Twin Strike at-will attack power) or multiple damage rolls, the bonus damage is applied to all relevant expressions involved.

However, any expression referred as extra damage is itself another modifier to a damage instance. See Rules Compendium, page 223.

Extra Damage Many powers and other effect grant the ability to deal extra damage. Extra damage is always in addition to other damage and is of the same type or types as that damage, unless otherwise noted. Because of this rule, an effect that deals non damage cannot deal extra damage. However, a power doesn't necessarily have to hit a target to deal extra damage - it needs only to deal damage to the target.

Under this category fall many Strikers' features, such as (but not limited to)...

• The Rogue Sneak Attack,
• The Ranger Hunter's Quarry, and
• The Warlock Warlock's Curse.

Hunter's Quarry may be used once per turn; let's suppose the Ranger uses Twin Strike, hits both times, applies Hunter's Quarry (1d6) to the first attack, and benefits from his ally's Warlord's Strike for another +4 bonus damage.
1^st hit: he deals basic weapon damage [W] (damage roll) +4 (bonus damage) +1d6 (extra damage).
2^nd hit: he deals basic weapon damage [W] (damage roll) +4 (bonus damage).

Note that the extra damage from Hunter's Quarry is not modified by the Warlord's bonus: it is a modifier itself. For many aspects, you can consider it a still-to-be-rolled static damage bonus.

This depends entirely on how often your warlord plans on attacking, charging, and to some degree, how your party is built.

If your warlord is lazy or mostly lazy - as in, the sort who grants attacks more than half the time - neither of these is a particularly good weapon choice. You might look at something like a chieftain's weapon, or something else that boosts your granted attacks.

If you have other cold-based characters in your party and your warlord will be attacking regularly, then the perma-frost combo (frost weapon + lasting frost + wintertouched) is a good choice. Frost weapon and lasting frost are both classed by The Warlord's Handbook as light blue in general feat rankings for warlords. Wintertouched is black, but that's partly due to it's uselessness prior to late heroic or paragon. (In character optimization threads, light blue is the second highest rating given to items, whilst black is mid-range).

If you don't have other cold-based party members, or if you are otherwise fairly optimized for charging, then the vanguard weapon is a good choice (are you going to charge several times/encounter?).

Other things to consider would be if one of your party members is going to take the Morninglord paragon path, then a weapon that deals radiant damage should be on the menu (Sunblade, Radiant Weapon).

There is not a right answer here, and as you are a leader it largely depends on your party composition and how you perform your primary role.