If I listed every "issue" I have with TS3 I could fill up a whole notebook. If it weren't for the awesome modding community I would never have played it. I'm on record multiple times in these forums saying as much. And yet, I did play it for the past 5 years, and enjoyed (almost) every minute of it.
I totally agree with your comment regarding the fact that many of my complaints about TS3 appear to have been addressed by TS4 but that it is disappointing that we have to sacrifice so many good things in the process.
The things from TS3 that I will actually miss is a pretty short list though:
CaST
Toddlers
Family Tree
Pretty much for me, that's it....and I have absolutely no doubt that 2 of those 3 will come down the road (possibly all 3, but I'm not as certain about CaST as the other two).
In the process it looks like I will get sims that are actually interesting and will interact with each other and neighborhoods that feel more alive when I go to public locations and worlds that are interconnected so that every time new worlds are added with EP's I can just keep playing the same family in the same game that I had been (the lack of this feature being the thing I most hated about TS3).
I'm also expecting that when the EP that adds dating features and what not comes out (ala Hot Date/Late Night) it will actually be GOOD, unlike the lame version of dating that is in TS3. And the same for vacations, and celebrities, and entertainers, and hobbies, etc.
For so many of the expansions, they were just so...blah...in TS3 compared to previous sims games. The way TS4 goes back to being more about the Sims than the World the sims are in gives me very high hopes for how these features will be handled in this new game once they are added.
Regardless, I have no intentions of uninstalling TS3 when TS4 is released. I don't need to compare them to each other, I will play whichever takes my fancy each time I sit down and play.