AC vs. Accuracy

[Jorgo]

So it seems like for players and monsters, AC goes up very little, if at all, after the first few levels. However, accuracy continues to increase. Do hits become way more common or am I missing something?

[LibraryOgre]

No, that's pretty accurate. For fighters and clerics, you'll see a slow creep in AC during the early levels but, after about 5th level, your AC only really goes up with magical treasure. Thieves, druids, and mages have even less of an increase... your thief probably won't go up in AC until he gets magical armor, since the defensive non-armor tends to go to the cleric, and anything magical that works for druids is usually an oversight.

[Thrudd]

AC doesn't go up, but hit points do, and so do saving throws (well, they go down, technically). Really powerful monsters sometimes also have resistance to non-magical weapons, maybe magic resistance/spell failure chance, an straight up immunities to certain types of spells. Hits happen more often, but because damage doesn't go up much you are much safer than you were at low levels.

[CE DM]

It depends what game/edition you are talking about.

Magical gear gained (or lost) can greatly affect AC in AD&D or classic D&D games, and this is much more the case than at low levels.
As levels rise, everyone generally aims for an AC of 0 to provide a base defense, lest they be taken out easily by mobs of low level types. AC 0 provides only mediocre protection vs potent monsters, though, and next to none vs the mightiest. Warriors (and other armor/shield users) can achieve AC 0 fairly easily, sometimes right at L1, but the other classes need specific magic items. These often are not as hard to acquire as some think, however, as NPC foes using them make them far more common than the tables would otherwise suggest. AC -1 to -10 does tend to provide quite good protection even from relatively powerful monsters. What AC generally cannot keep pace with is the warrior class's ever increasing accuracy, and that's intended, if a bit of an issue at very high levels.

For d20, it's a slow climb as $ goes to magic, & thus AC, but the accuracy gap is intended to fuel feats that mess with damage &/or accuracy (power attack, expertise, etc) as well as iterative attacks being 5 points worse at each new attack (EX base +12/+7/+2 for L12 warrior classes)