This is in part my feelings on the matter. This is why I started the thread.For me, the main issue with mounts, as a referee, is if a player has their character expend a fair amount of "things that make a character special" in order to have a mount and then only gets to use the mount irregularly. That forces the player who wants to invest in a mount to give up a lot during the many encounters where you can't use the mount.
In older systems where there might be a skill, multiple mounted combat feats and a character class with a mount class feature, if they took several mount feats and were a paladin but only got to use the mount one in three times, it left them disadvantaged compared to other players who focused on more relevant ablities.
Sure, a player could just live with that but most of my players were not willing to leave that much on the table and mounts didn't get used.
See, this is something I don't accept. So what if 50% of areas aren't horse friendly - that's still HALF that you can use in your adventures.So, what do you do when you have a standard D&D campaign where 50% of the terrain is mount unfriendly? (Note, I'm picking 50% as an entirely arbitrary number, and is not meant to be indicative of anyone's campaign specifically - pick a number that works for you if you don't like 50)
Again, you are the DM, you have the ability to set the ENTIRE CAMPAIGN in the plains if you so wish.