I didn't understand the importance of a  "family-friendly hotel" until my son was 6 months old and we took our first vacation.  We stayed in a regular hotel room and it was a nightmare.  We ended up on our son's schedule; napping in the afternoon and bedtime at 7 pm. 

I've since gotten much smarter and in part due to Ciao Bambino, www.ciaobambino.com, a website that focuses on family-friendly accommodations and family travel tips.  I may not always have a separate bedroom but knowing that my room has a balcony where I can escape during nap time is a plus.  The site lists important information that can sometimes make or break your vacation - it was really helpful to know that some of the condos I was looking at on Maui had stairs and no toddler gates which lead me to choose different accommodations - useful information that the guide books do not provide.

As my kids are getting older I now look for different options.  A pool is still paramount in our household but having a kids club option is something new that we are trying out.  It allows us to possibly have a night out or some time for ourselves and the kids love it.  The only caveat can be if you are traveling off season because sometimes your kids may be the only ones there and then it's not as exciting.

Ciao Bambino has a blog too that is well written and interesting.  I particularily like the Top 5 Kid Activities articles for cities.  I refer to these often for new ideas when our family travels.