Our fighter was dropped in the KotS encounter with Irontooth. She viewed it IC as a 'failure' and is now dedicated to being a damage sponge. She took Unstoppable, has Comeback Strike and just took Toughness. Her surge value is 14, and she has 11 of them. Sheesh. If you consider surges+hps, she can soak up over 200 hps in a day, at 4th level. I think they're OK as far as being able to take damage goes.
As far as AC is concerned, a defender's AC can be /too/ high. If it's substantially more than 2 higher than those he's protecting with his mark, it becomes worthwhile for enemies to completely ignore him - 'Challenge' damage or no - and that's actually /failing/ at being a defender. Shield Defender AC is trivially 19 or 20 at first level, and goes up from thier based on magical armor acquired. Striker AC is likely 16 or 17 at 1st, assuming an 18 DEX, and only keeps pace with heavy armor if DEX (or INT, for a Warlock) is boosted at every opportunity. So the defenders have 1-3 AC on strikers.
Leaders can get defener-like AC with heavy armor & shield feats, but the preqs can be tough to scrape together. A 'lazer' Cleric can prettymuch forget it, and a Warlord or Battle Cleric has to balance investing in enough CON for heavy armor proficiency vs secondary and other tertiary stats, so it's not an easy thing. Armor specialization for a leader can be a pain to qualify for, too. My Warlord won't have armor specialization until Epic level, for instance, while his Fighter ally will easily qualify at 11th.