For instance, while Improved defenses is nice, I think the Great (and especially Superior) Fort/Reflex/Will feats are even better. Let's face it, my dwarf who has nothing but 1/2 bonus to his Reflex, is pretty much always gonna get hit by an attack targetting reflex (even with a +1). But give him another +2 to his Fort (and combine that with a resist to ongoing damage to boot) and suddenly he's saving a lot more damage on him.
Superior Defenses are often better because of the rider defensive effects which happen a lot more often. Unfortunately, they have a very high ability score buy in cost, so they are difficult to purchase. Most PCs cannot afford them and those that can tend to already have those particular NADs high.
But, the +2 Fort portion of Great Fortitude does little. Assuming that only one attack in six goes against Fort (3 in 6 vs. AC, 1 in 6 Fort, 1 in 6 Reflex, 1 in 6 Will), it doesn't matter what a given defense is.
Adding +1 to all 3 NADs is better than adding +2 to 1 NAD because out of every 120 attack rolls at Heroic, Great Defense will stop 2 hits whereas Improved Defenses will stop 3 hits. Stopping 2 hits out of 120 attack rolls is equivalent to stopping 2 successful NPC hits out of about every 3 levels (assuming a given PC gets attacked 5 times per encounter and there are about 8 encounters per level, Defenders might get attacked more, Controllers a lot less). Improved Defenses helps once per level on average.
Improved Defenses gets even better at Paragon by helping 2 times per level (stopping 6 more hits out of 120 vs. 3 more hits out of 120 of Great X) and Epic 3 times per level (stopping 9 more hits vs. stopping 4 more hits).
Granted, this assumes a fairly equal distribution of NAD attacks of different types and that each has a similar level of devastating effects if it hits, but as a general rule of thumb, that can probably be assumed.