The only real concept in here is that "silly one you did for easter" , its really great.. the other stuff is kinda , cliché and a little boring . A lot of flat areas ( not as in value flat but huge areas that is empty ) which you could fill with details and make the concepts more interesting
as for the stuff not being concepty enough or being flat design wise, yeah I'm aware of it. Currently really concentrating on getting used to using painter and rendering with it, so the design is taking a bit of a back seat.
Maybe I'll give myself more than an hour for one of these and push the design a bit more...
The bats look fine to me, I actually kind of like the way you did them.
I do sort of understand what they are talking about with the "flatness" A lot of the recent stuff looks like it was posed in fron of one of those photography studios backdrops, you know what I mean? Lots of diffused colors. I think if you try to give your bg's more depth it will help a lot. Remeber the distance haze. Values get lighter, and colors less saturated the farther back an object is.