When Ghostcrawler says that defense serves double duty he's talking about two areas. The first area is defense serving as a check against the level mobs you're fighting. If your defense isn't high enough, you're going to get your face crit off and eventually crushed. This is why level 80 tanks need 540 defense to go and tank mobs three levels higher than them.
The second area is defense serves a double duty in is its ability to provide raw tanking statistics. These include things such as block, dodge, parry, and miss. The more defense you have, the better those statistics are on your character. What these do exactly is beyond the scope of the article, but needless to say they're important to tanks.
So when GC says that these two combinations might be a little awkward he's talking about the interwoven nature of defense. However we can't exactly be sure what in this combination is awkward in his mind. My guess is that he's referring to this one "catch all" statistic that defense is. It is so important to each of those two areas that if a person wanted too, or needed to, focus on one statistic through gear it would be (relatively) more difficult to do so rather than focusing on them all. Further, having this catch all statistic serves to limit encounter design to some extent and presents odd situations.
An odd situation is a "boss" mob being three levels higher than you, yet tuned to level 60 damage. Ie: Onyxia.
The other insight Ghostcrawler gives into defense comes along the same lines, that it provides both mitigation and avoidance. As stated above, defense increases block, dodge, parry, and miss. If it only increase block they might have an easier time balancing classes and abilities across the board.
If you look at the four tanking classes (Druid, Death Knight, Paladin, and Warrior) you can see the different ways some of these statistics play out. Druids like to have a lot of dodge, but block does nothing for them. Death Knights like to have a lot of parry, but block does nothing for them and dodge isn't the most desirable statistic. Paladins are happy with block (to some extent), but they're not really avoidance tanks. Warriors can come up with gear combinations to use all three.
The above paragraph will probably get some loud and proud disagreement based on individual's tanking style and opinions, and that's okay. Nonetheless, it does show how the statistic could use greater fine tuning in the balancing act across classes.
It's interesting to note that defense serves the Warrior class the best in its versititly based on gear and ability rotation. It goes to show that defense as a statistic is more a throwback to vanilla WoW when Warriors were the de facto tanks.
With these bits of philosophical underpinnings in mind, Ghostcrawler does say that changing some things is a possibility. However a change to defense that would solve the above is something that would change the fundamental game mechanics. That is likely something we'll only see in the next expansion (if ever).