From what I've read above, the discussion concerning consistency seems to be centered on the idea that a rules-heavy system provides more rules for more situations than a rules-light system. In theory, these additional rules will give us more consistency in rulings than the average GM would on his/her own. And for many groups, I think that's accurate. But I also think that discussion only covers part of the issue with consistency.
Consistency is also dependent on the complexity of the rules elements. For instance, the Dodge feat grants a +1 AC vs. one opponent, selected at the beginning of the feat owner's turn. Many groups houseruled this to be +1 AC vs all opponents. Why? Because players and GM's too often forgot to select the opponent. Because it's too much work to remember this one feat for the small benefit gained. Because a +1 AC vs all opponents is hardly unbalancing for a feat.
Similarly, most races have bonuses to saves, AC, attacks, and/or skills - bonuses which only apply in limited circumstances. Quick, when was the last time your halfling remembered to use his +2 bonus to saves vs fear effects? Or the GM remembered to give your gnome a +2 bonus to his save against an illusion?
And then we have a slew of spells which produce certain effects, the exact nature of which requires a lawyer to resolve. We're still having discussions on these boards concerning basic spells, such as
polymorph, protection from evil, grease, and others. Many of these spells could have been written more broadly and simply, but fear that they would be abused apparently led to a confused tangle that players and GM's are still trying to figure out.
The basic fact is that much of the complexity in 3.x/D20 is not necessary. It's a design choice - we have
18 bonus types, rather than a half-dozen. Some stack, some don't. We have 8 item creation feats, instead of 3 or 4. We have spells which provide a narrow effect in special circumstances.
All of this complexity leads to inconstency in play. I'll bet any amount that my group is getting some of the rules wrong. And - with the possible exception of Hyp's group

- every other group is too. Heck, we have articles on the WOTC site, ostensibly to help explain the most problematic rules (Rules of the Game), which often show that even WOTC is often confused by the rules. This problem is not a case of needing rules for common or uncommon situations, it's a reflection of the unneccesary complexity in numerous elements within the system.