What is it about us that we always seem to compare and then worry when things don't match? I have to watch myself on this, too. :)
As far as children are concerned, I really started understanding this after having my second child. My son (oldest) was fully walking (let's face it, RUNNING) by 11 months of age. My daughter did not fully walk on her own until 16-17 months. I was totally freaking out. I think I even have a question here on Mamapedia about being worried that my daughter wasn't walking yet.
I think that to an extent, if something were truly wrong with your daughter, you'd probably have a feeling that something wasn't right. In the case of my daughter, I could look at her little face and KNOW that she was comprehending things. She understood things. She was bright and happy....but by golly, she wasn't interested in walking. Of course, now you wouldn't know that! She is a rambunctious 3 yr old!
On the flip side of this, I remember worrying about our boy, because he would not sit to read books with me. We'd get about 2 pages in, and he was done. I was a first time mommy with tons and tons of books, and my son could care less about the books! For this reason, I didn't even spend much time trying to read books to our daughter until she was closer to 9 mo - a yr in age....we'd sing and talk (jabber) at one another. Low and behold, we started reading books and she would sit there patiently, turning the pages with me and studying the pictures, etc. I remember thinking that I was so stupid for assuming that she wouldn't sit still for books just because her brother wouldn't.
Part of the beauty of those gazillion trips to the pediatrician in the first 2 years is that you can ask the doc about these things. If they are truly worried, they'll tell you. They know enough about kids to be able to say if something needs further observation.
Get yourself a book on childhood development (I also have the What to Expect Book the First Year book that another mom mentioned) or subscribe to a website like babycenter.com, where they send out weekly development newsletters. I think you'll start to see that 'normal' can mean so many different things. Love your daughter...it sounds to me like she's doing fine. Enjoy this time and let things happen when they will. Talk with the doc on any major concerns (but write them down, because I promise you'll forget when you get to that appointment). :)