The "At War" check box allows you to attack members of a faction if they are not hostile toward you. Check that box and you can now attack them.

You get a 10% discount at high reputation for buying items, item repair, training costs, and taxi flights.

All members of both the Horde and Alliance are reporting low stockpiles of various textile resources! Donations of cloth are now being accepted in the following locations: Darnassus, Stormwind, and Ironforge for the Alliance; Orgrimmar, Undercity, and Thunder Bluff for the Horde. Seek out an official Cloth Quartermaster for more information. You can use these quests to increase your reputation with various major factions to allow you to ride mounts of another race.

There is a spell effect when you increase your reputation to a new level.

The Reputation tab displays on mouseover exactly how many reputation points you need to attain the next reputation level with that faction.