There's a good thread in the WoD Beta forums discussing the current difficulty level of the Pet Menagerie. Basically, for a plot that everyone will want to have at at least tier 1, the entry level for it is too steep for WoW's general audience.
Currently, to finish building it, you need to complete the quest Pet Versus Pet. I detailed what the objectives are for this quests in this post. For a veteran battler, a task like this is something that can be easily handled. Even if a team and strategy aren't planned out from the get go, a long-time battler generally has a large selection of pets at their disposal to try out. On top of that, vets have the knowledge to back up their massive armies.
For a more casual battler or someone that isn't into Pet Battles at all, this challenge would probably seem impossible. Howl bomb strat, what is that? Yet without defeating all three legendary pets, the menagerie cannot be fully built. It will still exist in its assigned location, and you'll see the NPC Lio the Lioness, but imagine how irritating it will be having just ONE plot forever incomplete?
So with this in mind, I agree with many of the points made in this thread. If you take a look at all of the secondary profession plots, the quests necessary to finalize the plots and bring them to tier 1 are straight forward and anyone could complete them, regardless if they're proficient in said secondary profession. Shouldn't the Pet Battle plot be treated in the same way?
It would not only satisfy the overall completion of the garrison, but could also be a gateway for players who never really picked up Pet Battling. It might spark an interest or curiosity to check out just what battling with these little pets is all about. A lower entry level means a wider audience can be reached.
By no means is anyone saying that Pet Battles should be "dumbed down" though. There are some great ideas for gradually increasing the difficulty level for each tier of the Pet Menagerie. Introduce more difficult quests over time to finalize the building of each level. Present a challenge that can only be completed using the daily trainer battles. Maybe even specific achievements need to be satisfied to prove that players do know what they're doing, and are ready for something harder.
Anyway, it just makes sense to remove the high difficulty level for simply building the plot. Most probably won't take part in the daily challenges or have any real desire to do so. But at least their garrisons won't be incomplete, and simply having the menagerie plot unlocked and sitting there might make them curious enough to try it out.