1) Most pvp players and guilds switch to alliance thanks to mercenary mode, imbalancing wpvp and making being horde meaningless. Which raise the question, why was it implemented in first place?
2) Mercenary mode is avaiable only for alliance---> horde. This means that EVERY premade, alliance or horde, is always into the horde side. This make bgs for alliance solo players, hell.
3) Most Blackrock players who plays BGs, are testing/trying a class or pvp for their first time, and it's obvious. Most of them are alliance, and will be in the alliance team. This combined with point 2, make bgs double hell.
4) Not everyone likes mercenary mode, not everyone want to play a different race. Someone likes being into a faction, the lore, and a healthly rivalry between the horde and the alliance. So the solution is not "just activate mercenary mode".
5) Mercenary mode brings some bad side effects, like bugged pvp achievements, and some of them are very difficult to revert; not knowing what to expect while fighting in bgs, things like a Tauren shadowmelding, expeting the silence from a belf which he doesnt have because human, and so on.
6) About Cross-realm, I have yet to meet someone who actually likes it. Icecrown players never needed it, and now they have to deal with BR noobs in bgs and BR scripters in arena. The vast majority of Lordaeron players seem to be against it, and for good reasons. Blackrock legit players also were more happy with their own server, 264 meta, transmog system, and so on.
I'm thankful and I respect the hard work that Warmane surely have done for implement these systems. But unfortunately, they bringed more problems than anything. Please listen to the community feedback and be open to reconsider some changes, either enhance or (better) remove cross-realm and mercenary mode.
Because the general consensus is that the pvp scene was way better before.