Hmm... I actually like that you can just buy (or rear and train) a decent mount, complete with a bunch of templates. And although I don't have a problem with it personally, some people will probably object to basing the strength of the Magical Beast Mount off of CR when all other cohorts are based of ECL.
Also, why would you take Warbeast Mount when you could take Magical Beast Mount? The second one seems superior in almost every way.
Well, I LIKE it, too, that you can do that, but, for instance, im my current E6 group the mounts people could have bought at level 1 (they did have more cash, but only 1 player did buy a mount) totally blew all the PCs out of the water. I was basically building fights to challenge that one mount (A magebred warbeast Riding dog.)
So I was thinking of a way to at least make them more "expensive" in a much rarer commodity in D&D terms, i.e. a feat.
I do have a simpler answer, too, though: We all know that Druids blow lots of things out of the water, even without their AC. So it shouldn't be too much of a stretch of just using that, well established, mechanic, especially the advancement. Hence, how about this feat:
Animal CompanionPrerequisite: Handle Animal 4 ranks
Benefit: You gain an animal companion as a druid. Your effective druid level for this companion is your character level minus any levels in spellcasting classes that eventually gain 9th level spells. This feat does not stack with other abilities that grant or advance an animal companion. Your effective druid level for this animal companion can never exceed your character level. This feat qualifies for feats, classes and abilities in the same way the druid class feature does.
Special: If you are a Ranger, exchange the ranger class ability with this feat at your discretion.
How's that?