Middles vary just as much as littles do. I identify as more of a middle than a little because I'm very independent and have interests in more "grown up" activities as opposed to little activities.
My daddy and I have been building cute models together. (It's like this one!) I like to colour but I have more "grown up" colouring books. Some of the mandala ones are gorgeous and I know there's a Harry Potter one out there... Anyway, I think you see my point. Same sort of activities but just a more grown up version because, as a middle, I'm capable of more than a little might be.
Middles are bratty. There, I said it. However, I'm a very quiet and submissive person so it's a bit of a non-issue for me. If you think of that age bracket then there are more attitude and behavioural issues, I guess, but that will depend on the personality of your middle. I think middles that are sexual might be more aggressive than littles. I don't know, I can only speak for myself, but the name nymphet is out there for a reason. Perhaps there's a bit of a cross-over there.
On the whole, your middle is unique and special. You'll learn what she likes and what she doesn't like. Just make sure you have a conversation (indeed, more than one conversation!) about all the usual things including rules, punishments, rewards, etc. and I'm sure things will be good.