We all know that gems are great. Not only should you socket up all the gear that has sockets, but you should normally choose gear with sockets over gear without sockets because it ends up being substantially more powerful. That's not what I'm talking about. I mean that the concept of gems, while it sounds cool, just isn't working, and I think this is just going to get worse in Cataclysm.
A recent blue post confirmed that the "cost" of sockets in an item's budget is taken out of the most attractive stat on that item. Thus, tanking gear with sockets should have less stamina than tanking gear without, plate damage gear with sockets less strength, leather with sockets less agility and so on. But of course they really have more of these things. They have more because that's what you socket them for. This is why we prefer gear with sockets to gear without, because we can maximize the one stat we want.
And so the problem is really that there is one stat we want. Almost every class and spec has a stat that is better than the other stats by a fairly significant margin. Even if they have two stats that are about equally good there is always the plateau of high stats followed by the drop off to stats that are worth two-thirds, half or even less than half as much towards their main function.
Suppose you have one stat that is good and every other stat is worth at most two-thirds of that stat1, and that you can get a stat that is two-thirds as good in both of the colours that don't have your main stat.
Aside from heads that have metas, the typical pattern for socket bonuses is that matching a single socket gives a +4 bonus, matching two gives a +6 and matching three gives a +8. Epic gems are worth 20 stat points. How extreme do circumstances have to be for you to gem outside your best stat?
A single socket item is only worth matching the socket if you get your best stat out of it. So if your best stat is strength, match a non-red socket only if the socket bonus is strength.
A two socket item you should match only if one of the sockets is the same colour of your main stat. So if your best stat is strength and the item has a yellow and blue socket, you should just gem strength even if the socket bonus is strength. If one socket is red to begin with then matching the other is good as long as the socket bonus is one of those two-thirds stats.
A three socket item should be matched only if either one of the sockets is the same colour of your main stat and the socket bonus is your main stat or two of the sockets are the same colour of your main stat. So match gems if two of the sockets are red and the socket bonus stat is useful or if one of the sockets is red and the socket bonus stat is strength.
In any other case just gem strength.
And this example is designed to be very favourable to socket bonuses and to variety. In reality most dps wouldn't value any blue stat at even close to half of their main stat and most tanks wouldn't value any stat at half of stamina2 let alone two-thirds. This means people gem all stamina, they gem all strength, they gem all armor penetration. They use other colours of gems to match sockets only to turn on their meta-gem.
Sockets on dps plate might as well just be replaced with twenty strength and we'll be done with it. Sockets are little more than a very complicated system to let people trade in small amounts of a stat they really want for small amounts of hit to get as close as possible to the hit cap without going under.
With a number of the changes in cataclysm - such as clearly defining and accepting the idea that each class/spec has a best stat and the coming rating changes - I think we have every reason to believe that the difference between your best and second best stat are going to get even more extreme (with the possible exception of tanks). This will mean more gemming for your best stat no matter what type behaviour.
In order to redeem gems stats need to have a relatively equal value. One stat will always be better, but it should not be half again as good as other stats. Without that, gems will slip more and more into just being passive stat bonuses by a different name. Gear with gems lets people trade gold for item points.
1. On an item point basis. For example, this would mean that if your best stat was Strength then either haste or crit would be two-thirds as good and a blue stat would be two-thirds as good. If your best stat was stamina then a red and a yellow stat would be as good as stamina point for point, since stamina only costs two-thirds of other stats in terms of item budget. Similarly, if your best stat is spell power than a yellow or blue stat would be more than two-thirds as good as spell power. You can see from these examples that this is an idealized situation that is making stats seem more balanced than they are. The reality of stat imbalance is much more extreme.
2. Valuing a stat at half a stamina item point wise would mean valuing it at three quarters of a stamina point for point wise.