For me gameplay is doing fun activities with my Sims. Island Living and Snowy Escape are my favourite packs because of the fun activities. I have a lot of fun playing mermaids, building sandcastles, collecting seashells, and swimming in the ocean. For me Island Living adds a lot of gameplay. Now I can have my toddlers do something fun too (play in the sand, build sandcastles, and play in the ocean). Snowy Escape also added a lot of fun gameplay for me. I have a lot of fun having my Sims snowboard and really like that it's on an actual mountain with plenty of options on how they go about snowboarding down the mountain. This is way more fun than a halfpipe. I also enjoy the sledding and hiking. I really enjoy watching my Sims do fun activities.
I never really took note of whether the Sims have unique personalities or not (too busy doing fun activities), but I would like to have likes/dislikes, favourites, interests, and hobbies for my Sims. So long as I can choose what they are.