Reasonable AC at level 1 is just 20. More than that is great, if you can get it, but anything less than that is a hindrance. Ranged/Casters can get away with an AC up to 4 points lower than that, but it's just a matter of time before someone sneaks gets past the front line and hits them hard.

Since attack bonus improves by +24 across 20 levels, your AC must improve by at least that much, or else you're falling behind the curve. That's not the rate of progression for a good AC, mind. That's the bare minimum to even be participating. If your level 10 melee combatant doesn't have an AC of at least 32, then it doesn't really matter what their AC is, because they'll go down within a few rounds regardless. Again, ranged/casters can get away with having an AC up to 4 points lower than that.

The obvious problem is that, while offense only requires a steady stream of ever-improving magical weapon to keep pace, defense requires magical armor and shield and ring and amulet and at least one other source of steady AC improvements. (Personally, I go for feats.) Since keeping up with AC requires so much work, most players just give up, and rely on other effects (like displacement, or just having a ton of HP) to protect them. Which is sad, that the system would fail them so badly, but I can't really blame them for making the most of a bad situation. AC isn't even that reliable of a defense, given how many things can bypass it.

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