With the caveat I do not own the Sims 4 (I stopped buying EA products around the time Origin came out, although Origin wasn't the main reason) ------- what I understand is the issue isn't that the Sims 4 feels empty compared to prior games with expansions....
It's that the Sims 4 feels empty compared to prior BASE games.
I started looking up game features of just the BASE games, and you can see an obvious improvement largely from game to game, from Sims 1-3. There's not clear cut improvement for 4. There are SOME improvements -- beyond the obvious graphics and UI improvements which every game has had incrementally -- seems like the AI is a little better, building seems easier, there's more skills. In other places it seems like where something was given, something else, often something significant, was taken away. For example -- their new Create A Sim is more intuitive and offers more specific tweaking than ever, and there's some more voices and things like walk styles, BUT they took away the massively cool and helpful customization tool that was Create-a-Style (and I have a feeling they took away Create-a-Style because when people feel like they can ubercustomize their clothes so easily, they might buy less DLC). Create-a-Style also applied to house/furniture design and its loss is huge. The Sims from I think the Sims 2 on offered pre-rendered color variations so the fact that the Sims 4 offers that (albeit in a prettier interface) is nothing new and is in fact an obvious step backwards. There seems to be smoother evolution and development and skills for child and older skills, but babies have turned back into an object rather than a character again (which has not been the case since the first Sims, a 14 year old game) and toddlers have been removed entirely (and with them a whole subset of objects, skills, and challenges available in the previous games). There are simultaneously more and fewer careers -- 8 career options, all of which split in two different paths down the line, so technically there's 16 career paths (more than in any Sim game) but they're really just more choices in only 8 fields (fewer than in any Sim game -- the Sims 1 came with 9 careers; the Sims 3 base game came with 11 careers, 3 of which had dual paths).
Emotions are only new-ish -- Sims have always had moods, and how they work has grown more complex in each game. In the Sims 3, I can look at my Sims aspiration bar and see where it is, hover over it and see the emotion my Sim is feeling (angry, uncomfortable, sad, excited, happy), and indeed those emotions influence what wants appear for me to choose from --- that's actually nothing new, the only new thing the way the Sims 4 does it is it's shifted the focus toward it even more, and made that interface easy to interpret. Those improvements are a good thing, but they're not actually NEW, nor as innovative or game changing as the marketing materials make them out to be.
In the Sims 3 BASE game I can do pretty much all I can in the Sims 4, save for some largely superficial changes to Sim design itself, and a few skills. There's still loads to do -- and the kicker is, there's a larger and more interactive neighborhood to do it in.
It's also not clear to me from what I've seen if Sims 4 has bug and rock collecting which was a Sims 3 base feature and was a cool way to get your Sim cash without getting them a job. Also, to Jim (if he reads these) or anyone else who's got the Sims 4-- did they keep skill challenges and opportunities? Those were both introduced in the Sims 3 and helped make even base gameplay interesting. I've seen that the Sims 4 has achievements, but those are for players, not the Sims themselves (in the Sims 3, earning skill challenges unlocked new abilities the Sims could have). I see the Sims 4 kept aspiration awards which is cool, but what about things that help you earn short term goals like opportunities? Or is the Sims 4 just like Sims 1 gameplay (work, eat, talk, pee, sleep) without challenges to push you forward?