Ever since early WoW has there been talk about min-maxing, best specs and best gear. Whether you are interested in this kind of stuff or not, it is clear that it is a big part of the game. Personally I've never been very interested in stat weights and BiS gear, simply because I am too lazy to care. Just as in real life I won't try to solve a problem until the problem arises, but some people try to make sure beforehand that they never even get into more trouble than they need to and know at all times what gear they aim for.
So what is this BiS gear anyway? The other day an argument arose on Twitter regarding this very question, and I thought if anything the argument made it clear that it's easy to talk about different things when we're talking about BiS gear. Especially regarding healing and tanking you might ask yourself - is there really something like BiS gear that is BiS regardless? Regardless of spec, other gear, raid setup, fight situation and other factors that undoubtedly play a role in your performance? And even if some stats might be the very best, are the differences big enough to warrant always aiming towards them or is there still room for flavor and playstyle?
Stat weighting does work somewhat differently between tanks, healers and dpsers. But eventhough healers generally are said to have more room to expand in a direction that suits their play style and raid composition, even dpsers and tanks sometimes have different numbers depending on what kind of fight they're looking at, and the kind of buffs that the rest of their raid group bring. So what does the BiS concept aim to cover? How can we possibly talk about one gear piece as the best of them all when there are so many factors to take into consideration?
First of all it's important to note that most BiS gear actually is considered really BiS when paired with the other BiS items. Eventhough an item is considered BiS it doesn't actually mean that it is the best choice regardless - it really does depend. BiS gear is part of a complete set that when completed is the best possible setup of stats for your role. Because of the varying factors that surrounds the final result of someones performance, it might not automatically be the best course of action to go for a BiS item. Although of course you have to have it to complete the set, it might sometimes be better to save a BiS piece in your bags until you can complement it with some other piece that will put your stats in proper balance again - the goal is always to have the perfect balance of stats.
In some ways gearing really is like putting some Feng Shui into your stat proportions, only stat weighting is based on solid, actual numbers that you can't really argue with. You can't say that haste is better than crit because "it feels better". The throughput of haste might just simply, mathematically, be better. This matters more to some classes than other though, and here is the main difference between healers and the other roles. Dpsers and tanks have to deal with their numbers in a way a healer doesn't necessarily have to. This is not to say that healers don't have definite numbers of throughput to which they can find a way to maximize their performance, but that the role of a healer can vary much enough to more often warrant other solutions (much like some dpsers tried to hit cap themselves for aoe fights, eventhough hit might have a low stat weight for their general dps). Healers more often have to be able to switch between aoe healing, point healing, healing while moving, healing while stationary, burn healing or conservative healing - all of which require different approaches to stat choices. Dpsers could probably do this as well, but healers can have a certain role in a raid group which requires them to focus into a certain stat priority, which dpsers rarely have to.
Personally I really do feel like you should take a look at your own role and raid setup as a healer and find out, not what feels best, but what seems to work best. Experiment and change your stats around to see what gives the best results! But back to the BiS gear.
What is it that makes an item BiS if it's so difficult to say that a certain stat is worth the most at all times? The concept behind gear is that the higher the ilevel, the more stat points Blizzard will have put into it. Just as an example Blizzard might have put stats worth 100 points into an ilevel 100 item and stats worth 150 points into an ilevel 150 item. To this you must add the quality of the item. An epic ilevel 100 item has more stat points than a green ilevel 100 item. So the first thumbrule of BiS is that the higher the ilevel and quality, the better the gear - simply because you get more stats from it. But what if the stats aren't what you need? Pvp gear has loads of points into resilience, which is considered wasted stat points in pve. The same would go with the dodge stat for a caster or intellect for a melee of course. But the same also goes with stats that are actually useful to the class. This is what you're talking about when you talk about stat weights.
Stat weights aim to tell you which of your class stats, say intellect, spirit, haste, crit and mastery for a caster, are worth the most. Which has the most throughput if you were naked and had 1 of either (that is the simple explanation since not all stats scale linearly and there are also cross-effects, but it's the gist of it)? By knowing this you can choose between an item that might have higher ilevel, ie more stat points, but that is less well itemized, ie has stats worth less in throughput, or the item you already have. The stat weights are what differ between classes and situations, since some stats might give you more throughput if you aoe than when you do single target damage or healing. People who really want to min max can aim to get a gear that suits each situation and regear/respec when they know that the fight requires more aoe or single target damage/healing. Otherwise you will have to find a balance that will make you do a good job regardless, and BiS gear is part of that balance. BiS gear is aimed at having you perform in current content as good as possible, assuming you know how to play your class. In the end, the goal is to get as many stat points of as worth a stat as possible.
Because of reforging, we are now able to convert a percentage (40%) of a less favorable stat on some gear into a stat we actually prefer. This means we will yet again look at ilevel and stat points on gear, because some gear are worth so much more stat points than other that they're better eventhough they might have a less good stat than the lesser item. Some items have bonuses that make them worth so many stat points that they're better nearly regardless of any outlying factors. The tier gears are designed this way. The set bonuses of the tier gear usually provide with a big enough stat bonus to make them worth taking regardless of what stats you actually prefer. The healing t11 4 sets for instance provide 540 extra spirit, which is more than 100 extra spirit, or stat points, per item. The same goes for most 4 sets, and some 4 sets are worth so much that you might not even want to take higher ilevel gear (like the 4set balance set, until they nerfed it). Therefore, tier gear until you get the set bonuses are usually regarded as BiS. Other gear that are BiS are either such that are the best itemized, ie stat weighted in a way that suits your class the best, and have the highest stat points - or that have high enough stat points that a reforging still will give you more of the best stats.
It might all sound like mumbo jumbo, but it's really quite simple when you think about it. It's a trade and you always want to come out with more than you had. Some stats are worth more and you want to make sure that whatever gear you trade for another gear will have you get more of the valuable stats as possible. BiS gear are items that, combined with other BiS gear will give you the most of the most valuable stat as currently possible. The value of the stat has been determined as which will give you most throughput overall - it does of course not take into consideration if you happen to have some weird, unusual role in your raid group, but is aimed at how most players are supposed to tackle a situation. Stat weights are made based on idealized fights and in a world like WoW where we are a bunch of numbers fighting numbers, this actually does work out really well to have us perform at maximum. But I still think the best way to use BiS gear is to learn more about your class, why they prefer some stats over others and when you have to forgo the stat weights. BiS gear is a recommendation, and a damn good one - but the best player is still one that knows when the BiS gear isn't BiS for them.