Sneak attack

Sneak Attack
A rogue may do additional damage when attacking an opponent if one of the following conditions is met:
 * The target is under the effects of a successful bluff
 * The target is helpless
 * The target is unaware of the rogue's presence.
 * The target is attacking another player.
 * The target is intimidated by another player.

Targets immune to critical hits (such as an undead, slime, construct, elemental, or anyone with high Fortification) are also immune to the Sneak Attack bonus.

Gameplay Implications Allowing Sneak attack to work on any target attacking another player is an important change to DDO from the PnP rules, and it gives combat a more MMORPG feel. Because of it, DDO rogues can fight rather like a rogue in some other popular fantasy MMORPG featuring dwarves and elves. As long as someone else has "aggro", the rogue gets a damage bonus of 1d6 per two levels. Even at level 1, that's as good as 16 strength, and it just gets better from there. It takes more than just 2 or 3 sneak attacks in a row for a monster to turn aggro on you, especially if another player is using the Intimidate skill e.g. an intimitank(which holds aggro for a minimum of 6 seconds on a success).

Although the +4 rear-angle attack bonus is nice, you can also Sneak Attack a monster who is running directly for you. And don't forget that the bonus works with ranged weapons too, when within 30' of the target.

The "helpless" target is also an important condition, because any kind of spellcaster (including not just wizard/sorceror/cleric, but also bards) can make an enemy helpless for easy killing.

In fact, because it is so easy to Sneak Attack in the middle of melee, there is little reason for a rogue to actually sneak ahead and try to backstab enemies from stealth. Unless traps are suspected, it's better for a fighter to go first, so he can draw the initial aggro and allow the rogue to start Sneak Attacking when the monsters arrive.