The first +1 from Agility is a starting bonus, not a "+1 per tier".
Its +1 per tier. Its just frontloaded slightly. The game is filled with abilities and bonuses that are +x per tier, with the +x granted at level 1, 11, and 21. Its the same as the scaling for non weapon non implement attacks, like the dragon's breath: stat +2 v non ac defense, improving to +4 and +6. Its obviously an effort to match the progression of a magic implement, except with everything frontloaded to the beginning of the tier.
Ah, I hadn't thought of that about Censure of Pursuit. I see its wording "moves away" as meaning "for any square moved, ends up farther", but any interpretation means ranged enemies are screwed. So the two avenger builds are quite unique!
We've had one in our game ever since the demo Avenger PC came out. The two builds are incredibly different. The Pursuit guy wants to keep himself an a single enemy far away from the rest of the fight. If his foe leaves, he wants to chase after him and drag him back out. He doesn't get the bonus damage for enemies fleeing him very often, we've found, but he does get to roll almost all of his attacks twice. The Vengeance guy is the opposite, or at least has the choice of playing the opposite. He can isolate a foe and roll his attacks twice, and he can use his various powers that penalize enemies for being near him to encourage his foes not to wreck his ability to roll twice, or he can intentionally get himself surrounded, use powers that penalize enemies near him, and get bonus damage every time someone hits him.
The only other comment I have regarding the AC issue is that its worth keeping things in perspective. A totally neglected non AC defense only falls behind by three points by level 28 in comparison to a non AC defense that had its associated ability score boosted at every opportunity. A character in hide armor who totally neglects his dexterity or intelligence score only falls behind by three points of AC in comparison to a character who pumps dexterity or intelligence at every opportunity.
The real issue, if it is indeed an issue, is that lightly armored characters can always have greater improvement to their ACs than heavily armored characters because lightly armored characters can take feats to turn themselves into more heavily armored characters, while the characters who started in heavy armor have already maxed out that route of AC improvement.
I'm not 100% sure that's a problem, though, because the only thing it tends to produce are strikers and controllers with defender level AC at the cost of some feats. The closest to abusable in the whole game is probably the Avenger, since he starts out the lowest. And even he isn't SO bad. An Avenger with a 16 starting dexterity or intelligence starts with an AC of 16. He can take leather armor, to give him a base AC of 18. Then he can take the Avenger feat to improve AC, which by epic tier gives him a base AC of 21. That puts him, at the cost of two feats, equal to a paladin who has spent one feat. He can still keep going, if he really wants, by taking hide armor and hide specialization, but those have prereqs he might not like, and he's not getting as big of a benefit out of them as he did from the first few feats. He might have other priorities at this point.
If his AC were significantly exceeding
other characters, I might worry. But instead its exceeding the expectations for an Avenger that you might have if you look at the character at level 1. That doesn't bother me as much. At the very least, you can't look at that information and know that the Avenger is unbalanced. A base AC of 21 is reasonable for an epic level character, we know because other characters have it, and many other characters are one point or so off from it.