This is one of those long lasting issues that was something I had blinders on for too long and it took some hard numbers for me to admit to the truth. With that short caveat let me explain what I'm talking about.
Everyone wants to be BIS
It's no secret that WoW is a game about gear. We might lie to ourselves and say that it's a social game and we do it for the PvP or the dungeons or the raids but the bottom of the line is, if you aren't getting new gear then it's a short path to no longer logging in. Stagnation in the game means people stop playing and the ultimate stagnation is when you have no gear to aspire to. So we all want to get the best gear possible which led to the idea of looking at what is the best item that can be obtained for your class for that slot, aka best in slot (BIS). Once you have that list of items, now you have a list of goals to achieve and people spend endless hours working on that list attempting to get to that point.
I myself have spent countless hours developing said lists and tools (soon tm) because I like to see upgrade options and enjoy exploring the odd items that Classic has in it. If you haven't seen them, here's the two I did for tanking.
Druid Tank BIS
BIS is not static
But in making these BIS lists I didn't and still don't like the idea of "BIS" as an all encompassing thing. There's too much variation in how buffs work and encounters from a basic standpoint to have a fixed list. Also and more importantly, BIS changes depending on what you have in your other slots. People's specific single BIS list is all well and good but it's tailored to itself. If you don't have every. single. piece. then there very well may be and are items that are actually better for you that you should know about. An easy example is hit. If you don't have hit cap or the items on the list that correspond to conveniently be exactly the hit cap you need, then your set isn't performing optimally.
If you want to be performing optimally you should take a smart approach to your gear and always look at how to optimize your set with what you have but also see it for what it is - a constantly moving answer depending on your encounter and setup. Only you can can know how long your guild takes to kill bosses, how much down time between pulls there is and the consumes needed to get the job done. These numbers will help you understand how to value stats like intellect for more burst mana needs vs pure Spell Power or even MP5. See the MP5 Topic Tuesday for more on that.
When you have a handle on your encounter you can start to make sense of the kinds of gear to shoot for. Don't stick with the set BIS list people show you. Explore what else is out there and what options you can look for that can adjust to your play style and your guild's raid style. At that point you will be in control of actually understanding what is BIS for yourself rather than copying what someone else says is best.