First thing I noticed was that I had no way to heal my pokemon. I was hoping to be able to just start playing. But that wasn't a big deterrent. I dug out a mine and after a couple of hours eventually found some diamond to build a healer.
Beds weren't an option because I started in the middle of a jungle with no way of getting wool. I had no easy way of healing (well, I'm assuming mareep is low level and should be simple enough to kill vs finding dimaonds). In addition, there are no apricorn trees in the jungle. Since that's where my spawn was and I have no way of making a bed to change my spawn, I had to wait even longer until I was able to consistently find my way back to where I'd set up my base of operations (as small and eager as it was).
So last night I got to play long enough to just make it into another biome and find a few apricorns. I was under the impression that two apricorns could be gotten from a tree--right click for the first one and chop it down for a second. I couldn't figure out how to get a second, so if someone can help me with that it'd be much appreciated.
So with my limited experience I've just a couple suggestions. Either make apricorns available in all biomes, or force the world's initial spawn to only be in biomes that can have apricorns since it's pretty integral to the mod. Since wool doesn't seem readily available to make beds, make another readily available way for the player to change their own spawn point. And have it easier to heal your pokemon from the start so that the mod can be played right from the start.
This is a minor thing, but if peaceful mode is enabled, I suggest making it so that wild pokemon can't initiate battles.
edit: Also, the "battle camera" spinning around is kind of offsetting. Don't know how to say it better than that, it just doesn't feel natural or like it belongs.
I should also say that I'm really impressed with what I've seen, I just wish I could've gotten more directly into it than having to do all this set up first. Now that I've got a healer set up I should be able to do more with the mod, and less with "vanilla minecraft" stuff (i.e. mining and building).