So many people are literally ignorant nowadays in why Balance is Moonkin based. It's not about enthusiasm, if that's so then your same logic should apply to Bear and Cat forms. That said, not sure how many actually got to see the old quests that explained said reason behind it, but check these quests here:
Favored of Elune? >
http://www.wowhead.com/?quest=3661
followed by
Moontouched Wildkin >
http://www.wowhead.com/?quest=978
Which eventually leads to
Guardians of the Altar >
http://www.wowhead.com/?quest=4901
and finally
Wildkin of Elune >
http://www.wowhead.com/?quest=4902
Since Cataclysm removed them, a lot of people scream about it's irrelevance because Legion made a massive parade on the Forms themselves, yet before Legion, the same applied to Bear and Cat Form... Tree Form was even worse than those two. |You hardly saw NPC's be in Tree Form and instead it was just Treants themselves. Out of all of them, Moonkin actually had a purpose. Others were quite literally based off of existing characters as a sake of "others could do it" like the the Druid of Claw/Talon/etc. or Wild Gods.
The only reason I can see Bear/Cat having little hate is because they make sense, they are real animals which automatically carries weight and assumptions that work well with it compared to a giant original Owl Bear, least Tree Form had the RPG famous element of Dryads. Including the imagery they convey, who doesn't want to be a badass Bear or Cat? It's been made more obvious with Legion's artifacts - which will be disappearing too ironically.
Tree Form was rather off if you consider that it's always been suggested that Nature is harmonious with healing. Pretty much every Form has a self heal - it's ingrained into Nature magic. Even Shamans use Nature Magic and all their specs can self-heal. Thus, why do you need to turn into a big are Tree to better heal someone when you already mastered the ways of Nature magic by being a Druid? I mean, come on, at max level you basically are a grizzled war veteran and an Archdruid.
It's most likely why Tree Form was removed, it wasn't really fitting anymore as the universe progressed.
The simple fact is, when one with Nature magic and trained in the way of a particular shape-shift, you become a form that best represents it and empowers you. It's also why things like Goldrinn and the Wolf-form in lore isn't in-game, Elune scorned upon him and Druids were forbade to be Wolves - hence the hidden artifact skin of Balance Druids.
Elune favoured the Owlkin and their cousins. They are the closest beings to interact with the celestial powers they can get.
It shares no difference with other forms prior to Legion and in truth shares the same lack of awareness all round on their lore.
If you're going to play a game that uses little lore originally to be a Bear or Cat and whine about the other forms then you really need to switch classes if you don't get why Resto and Balance have their ways on shape-shifting.