How many times do you see people in your guild or in the general chat asking if this or that is good for them, or whether item A is better than item B? My first thought, a lot, and it is no wonder. It takes quite a lot of dedication and literally many hours worth of embedded studing to get the hang of the math engine that works under the hood of the game. So far, and will probably continue to, despite improvements in patch 3.2, use an addon called Ratingbuster. It is a simple enough addon, on your item's tooltip, displays the changes that would occur if you swap the item for an old one.
It shows all the changes in a very easy to read formula but still, it is a juggle to keep up if an item truly improves something. It depends on your spec, your role and game style and often you are stuck on situations where you actually, cannot benefit from an item until you get another. This is most evident to me in tanking gear, which still continues to amaze me.
Unlike dps with hit rating, tanks cannot let our defense fall below uncrittable, which can happen a lot easier than you realize. For instance, I did the math on the tier 8 gear and the deal is, I must first get the specific pieces first, namely the ones with the defense stats and yellow gems, so I can maximize my defense before getting the pieces without either and have my defense blummet.
After all, new leggings might be a really nice improvement, but wearing them takes you below uncrittable, they are next to useless. So let me take this moment to say, I understand and feel your pain Sylly. The game continues to test us all, especially with the upcoming changes in patch 3.2. We just have to quicken our stride and pretend we're keeping up.