I've honestly never played the game without any expansions, though I can see how that might get boring a bit. Overall, I think it is a pretty good game if you're willing to shell out for a few expansions. Compared to sims 3, it runs leagues better on my computer...sims 3 can be clunky, even on my gaming laptop and with modifications to make it run faster. Multitasking is cool. And I didn't like the cartoony art at first, but after going back to sims 3 after playing sims 4 for a while, I much prefer it to sims 3 style.
That said, there are some things I'm missing from previous games. Open world is a big one for me, though I know the implementation of open world was part of why sims 3 runs so badly, and it would take a lottt of time and effort. Story progression isn't the greatest, but like with sims 3, there are mods (MCCC) to help with that. Also, some of the things that maybe I took for granted in sims 3 and sims 2 take WAY LONGER in sims 4--i.e., showering, cooking, eating. So if you get 4, I'd definitely get some mods that help speed all those up or else it might get annoying. Speaking of sluggishness, it feels like skills also take forever to level up to me (but again, mods can help you adjust that). I like CAS and the clothes/hair, so I don't play with much CAS CC, but as others have mentioned, there is no color wheel, which is a bit of a bummer. Although I must say I LOVE the gender options in sims 4, and the way anyone of any gender ca wear whatever. Toddlers are also great and super realistic, but that can also make them tiring...haha...build mode is pretty cool though if you're into that!
As compared to sims 2, I honestly sometimes get the urge to go back to sims 2 and binge because there is a lot of heart in the series, despite the lack of story progression/problems where breathing wrong on your screen would cause irreversible neighborhood corruption. There were a lot of little details in the sims 2 that I really appreciated, like couples cuddling in bed, children dancing on their parents' feet, pillow fights...Honestly if the corruption issues didn't exist, sims 2 would probably still be my main game.
But overall (tl;dr), I'd say sims 4 feels like a sort of updated sims 2 with better graphics, faster loading times, better game performance, and with some homage to the silly but intimate details of sims 2 (although I think there are some simple moments lacking). Fin. +1 to @Maggiedoll 's idea too, $5 a month for a trial run of the game might help you decide!