Well, with at-wills the avenger is not so special, but with any encounters and especially dailies they are a lot nicer. Yes, Twin-strike basically auto hits with one arrow every turn, but the Avenger nearly auto-hits with every pwoer he or she has.
You'd be amazed how far that doesn't go.
The Ranger's heroic tier encounter powers are generally around two attacks, each for 1[W]+Str/Dex, with an additional 1d6 if he hits with at least one.
The Avenger's tend to be around 2[W]+Wis, but with the ability to roll twice and choose the better roll.
The Ranger is far, far better. As in its not really even a fair comparison. If the Ranger hits with just one attack, he gets 1[W]+Str/Dex + 1d6, which is going to be awfully close to the Avenger's 2[W]+Wis, and has the same chance of happening. And then the Ranger gets an additional chance to hit twice, dealing vastly more damage.
Here's a good way to look at most Ranger powers- as if they said, "Make two attack rolls, and choose the best. If both attack rolls hit, deal some extra damage." Basically, the Avenger gets the damage expression of a defender, with an accuracy bonus. The Ranger gets the damage expression of a striker, with the same accuracy bonus.
The comparison just gets worse with daily attacks, where the Ranger exceeds pretty much everyone in the game.
Really, its almost unfair comparing the Avenger to the Ranger's damage. His accuracy might be great, but even adjusted for accuracy the Avenger tends to be the low end of the melee Striker damage pool. He makes up for it with control and durability, and the occasional (generally rare) bonus damage from Pursuit or Retribution, but if you're worried about Striker damage that's unfairly high, the Avenger just isn't the guy to worry about.
