Here's a quick FAQ which should answer most of the reasoning behind the change and whats next:
Have we destroyed PvP?
No, the numbers show that.
Player Kills Player
Tuesday July 25th: 21,697
Monday July 31st: 27,661
Tuesday August 1st: 22,796 (+5% compared to previous week, -18% compared to previous day)
So yes, these changes have had an impact. And we're looking very closely at them. But we still have plenty of players dieing to other players.
Have they destroyed solo PvP/Ganking?
Its hard to get actual numbers on this, but there's plenty of anecdotal evidence that its still possible: Solo PvP After Dismount Change
Of course this often relies on a mistake on the side of the victim, but we've always said this should be the case: No single player should be able to force a fight against another mounted player with even gear if both play perfectly. Theres still plenty of room for error as those videos show.
Have they made ganking solo players harder?
Yes, absolutely. We firmly believe this has to be the case, however, to maintain the PvE/PvP ecosystem. If killing unwilling targets becomes easier than gathering the resources for the gear those unwilling victims wear, then PvE becomes an unattractive activity. If PvE is unattractive, you will soon run out of victims and solo ganking will truly be dead.
Why did we do it?
Because we believe the previous balance was broken*. And we should already have fixed it in beta. We didn't see it in beta, because of the lower player numbers, but it became very evident with the large player numbers after launch.
*What we consider broken is the ability of ganking/gatecamping groups to catch any player they want as long as they have at least one faster mount in your group, by riding up to them and breaking their gallop. In particular the gatecamping situation was quickly escalating as camping groups were acquiring faster mounts and we wanted to put a stop to it before it got completely out of hand.
Is this final?
No. We're now looking at the impact of the change and we're now looking in more depth at the balance for ganking players. In particular we're interested to find and fix what else is broken.
We're currently looking at the mount speed progression, which makes players on higher tier mounts almost ungankable. We're testing whether it would be feasible to put most of the mount speed into the gallop mode to make all tiers of mounts similarly easy to engage while dismounted.
We're also playtesting reducing the skill cooldown after dismounting to allow for more skill based engagements after dismounting.
Do we still think we were right?
Yes. As developers we need the ability to change and improve the game. We cannot be intimidated by the perception of the changes we're making. And we also cannot wait for all changes to be perfect before launching them- because changes like these cannot be playtested on a small scale. We need to keep moving. Everything else would lead to a standstill. And why would you want a standstill when we've just started? Of course we're going to change the game, and of course you're not always going to be on the side that benefits from those changes. That is the nature of making an MMO: you have a massive amount of players playing together, and our concern has to be the well being of the entire community.