It was hard to predict and give feedback on immortal enh shamans when their healing/damage was nothing as it is on live. It was hard to see how dumb mage CC were when your teammates broke it instantly (and your damage was lower than that of most healers). Druids could outheal 5 people with rejuv->swiftmend spam (which was obviously going to get nerfed) so it was impossible to draw any conclusions about anything else with them.
In most arena games I played on beta you either faced really bad people or classes with obviously broken mechanics, neither of which was a solid ground for more than guesswork of what WoD would be like. There was no mmr system in place so even if you did end up queing with friends instead of randoms you would end up facing very unorganized teams which you steamrolled simply because of their lack of coordination.
The fact that Blizzard did not actually listen to and fix most real problems didn't really help either. As an example, we were pretty many complaining about how giving Silence to disc priests was just covering up the real problems with the spec but they went through with it anyways. Disc was carried by Silence at the start of the expac and the moment it got nerfed they disappeared from the ladders.
Beta keys were also given to most people that had played the game for a long time, most of which were not high-end PvPers. The main source of feedback was NOT coming from Gladiators but from low rated players that just happened to have played the game since forever. All you need to do is to go to the official forums to see the level of quality of most of the feedback.
All in all, the beta servers were not set up in a way that made PvP testing really possible. We would have needed rated PvP as well as some sort of group finder system (like the one they have in the game now) to actually find decent players and play decent comps.
Of course the second problem (and most likely the reason why they didn't do this) is that in order for that to work, a lot of people need to play. The sample size of players with any form of arena experience, even 2200+, would be very small on the beta so the amount of good feedback would be very limited (and the queues long for the few good people). Unfortunately it is the only way if they want to get something out of PvP beta testing.
Edited by ottishen, 16 October 2015 - 12:47 AM.