Yep, I don't think couples talk specifically enough about children before getting married. Like you, my H and I both wanted to have kids, but I didn't realize he meant that he wanted kids while we were young (in our 20s), while I wanted to wait until my career was established before becoming a mom.
What issues do you see with becoming parents? IMO there's never a great time to have a kid -- they are expensive (average cost is $300,000 to age 18!), demand attention, and force you to adopt a new demanding role as parent -- but I wouldn't trade my kids for anything (career or $$). It wasn't until I became a mom that I truly understood why parents are willing to run into a collapsing burning building or jump in front of a moving truck just to have a small chance of saving their kid. But, I would totally do that for my children (and so would my H).
We are still young, mid twenties. I guess it would just be such a change of lifestyle, we are happy, I guess I'm a little afraid to change things up, but she really wants to start a family soon. I do too, but Its such a big deal