Well, this one’s a bummer for those Riders of the Apocalypse out there.
The Horse Is Way Too Pale
The 
On a Paler Horse Hero talent for Death Knights is our first instance of permanent mounted combat. However, its usefulness is pretty limited in the giant zones of the War Within, as mounting your Skyriding mount is usually faster. However, here comes the Siren Isle, where there is no flying (yet)! What a perfect place to use your cornerstone Hero talent, right? Wrong.
As spotted by Varanae, it seems the talent is flagged to only work where flying is active, as it is not functioning in the one place it would be perfect for. Now, perhaps this is very much intended. It could be considered as giving DKs an advantage in the Siren Isle. Or perhaps it’s just an unintended consequence of the coding being related to flying zones. Either way, it’s a pretty big bummer for players who were looking forward to getting the most out of their Hero talent tree in the patch.