With hunters, warlocks, and unholy death knights, there are a lot of talents and glyphs that affect the scaling of stats to your pet. If you allow certain stats to scale too well, you suddenly unbalance the class. If the stat scales poorly, then the class falls in the other direction. Every talent and ability that taps into these has to be checked to make sure it doesn't throw things out of whack. Considerations on diminishing returns on certain stats scaling would also need to be checked.We'll try to do what we can. Technically it's just more challenging than you might think. For Cataclysm, we have on our list that 100% of stats scale. If they don't then certain stats just won't be as valuable for pet classes. At the very least, we can do stuff like convert your X into damage for the pet so every stat is valuable. Getting everyone to scale with every stat better is a major goal for the class team for Cataclysm.
Overall, there is a lot of number crunching and statistical analysis involved in trying to find the right balance between all of the knobs that they need to adjust. However, the frustration of out-gearing your pet will hopefully be behind us either in an upcoming patch or (more realistically) when Cataclysm hits.
World of Warcraft: Cataclysm will destroy Azeroth as we know it. Nothing will be the same. In WoW.com's Guide to Cataclysm you can find out everything you need to know about WoW's third expansion. From Goblins and Worgens to Mastery and Guild changes, it's all there for your cataclysmic enjoyment.