I have to disagree. In many D&D games, the AC is benefitted from Ability boosting magics, from armor boosting magics such as bracers of armor, natural armor boosting magics, and many additional sorts of dodge bonuses.
In d20 Modern, you really only have three types of bonuses: Ability, dodge, and equipment. Discounting magic and FX, it rises a good bit slower. Counting the FX stuff, it starts to even out again.
Consider that the highest equipment bonus is +9 (from the hostile entry armor, or whatever it's called.) then, the Defense bonus only raises as high as +5 to +12, depending on class. Add one or two points more due to feats, and that's still an armor class in the high 20's to low 30's. Some D&D armor classes (if power-gamed to the max) can reach the 40's to the 50's. (I know about the Smack-downs with characters of AC 130 or so, but most people don't pull out every trick in the book.) So in the end, there is a disparity. Most d20 Modern characters of 10th level or so have an AC of maybe 22 ?