I've always kinda thought it was the existence of the Artian weapons that was the big whammy that ruined it for me and my group. Thinking about the mount though, it did make it so you're totally disconnected from the world other than riding to your target and grabbing things with slinger along the way.
Meanwhile, in World I knew those maps inside out. The first time I fought Rathalos I was losing to the timer as much as his skillset (first MH game for me) because I couldn't figure out how to get back up to him. Needless to say, I learned the Ancient Forest really well.