As others have pointed out, it really depends upon the pack.
With some packs, I'm admittedly still exploring some of the "new" content that came with them as unfortunately, their releases often coincide with when I'm at my busiest and don't have as much time to play or I have to wait for sales in order to afford some of them.
I love most of the new worlds we've been given (especially as someone who is really into building and decorating)--but I can also appreciate it when we receive a pack where there isn't a new world, but lots of new gameplay or just content. In those situations, it gives me the option to really explore what's new and focus on that and my existing households.
I can also see how/why it would take the devs so long with each new world as I can imagine (outside of the length of time it must take to build/decorate lots or do the animations or create hidden lots) it must take a great deal of preparation to come up with them and their respective concepts to say nothing of having to map them out and deal with all the logistics and then the technical aspects that must come with it to say nothing of the graphics. I don't know how many people are on their team at this point, but that has to be resource intense on its own!
