Scrusi is unhappy with sets. They are too easily just a pile of math.
I have an easily solution for that: Make them piles of EVIL MATH.
That's right, sets that harm you. Want a collection of armor that looks good together and synergizes for a greater bonus? Great! This does that, except the bonus is a bonus of pain. Let's consider a full set of tier 50, which I assume that will be out within an expansion or two, in this case being the extra-heroic 25-man achievement set awarded when running with 20 people on an odd day of the week, with no higher than 5423 average item level. This set shouldn't grant +672 attack power. Instead, it should grant a 1% reduction in damage output.
You might say "but isn't this still just a pile of math, comparing the a full set with most of a set plus non-set gear?" And I say NO. Because you used the word "just a pile of math."
First off, imagine the theorycraft potential for picking which piece of the set you won't use. Imagine if somehow the entire set is still better; it will be no time at all before you get an upgrade, such as anything other piece in the game.
Second, this set finally has a negative effect. It has been far too long since WoW used negative stat modifiers. Vanilla, being the most sadistic of the flavors of WoW, had them all over the place. There was an awesome sword with -stamina, which also happened to be hard to get in a place that was hard to get to, from a boss event that was hard to beat, needing a class mix that was hard to make, wearing gear that was hard to get. This sword knew evil. It was a crowning achievement and it was NOT GOOD. Other items included a cloth chest, presumably for a caster, which if I remember correctly, had a big chunk of negative spirit on it. That just screams evil. Oh a cloth robe, you think this is for you, a cloth-wearing class? WELL HOW DO YOU LIKE THIS NOW!?
Finally, and possibly most importantly, it punishes people for trying to look cool. Vanity, aka pride, is one of the seven deadly sins, and while those are bad enough in the real world where some people, such as myself, are truly awesome, in a virtual world it just screams "nerd who is going to burn in hell forever; see Dante's Inferno for more information, which of course being a nerd, he probably has somewhere, downloaded illegally to his e-reader which isn't a Kindle because Amazon only sells Kindles to cool people." My point is that games have strayed from a strong moral foundation, with too much emphasis on killing and self-gratification, when they should instead be about humility and causing non-fatal harm to non-believers.