It's the friend aspect - I cannot make 20-odd friends for my Sim.
It was a problem to me too, make shore you have Attractive LTR and Legendary host, trow a few parties, tell few jokes and gossips and you'll have them in no time. But I still didn't take photos of 60 unique Sims for photography challenge, and never got some of the rare photos for collections.
I also haven't played all part time jobs, (Grocery and Movie) and now that I think of, I haven't played vampires more than a day for testing purposes. I didn't even like the dark need bar.
I haven't got Celebrity Gnome.
I haven't even tried The Gold Digger LTW. Others I think I did all. Even Private Museum and Famous Photo Collection.
And I forgot about PI, I didn't finish that one neither. But first thing I did when I got Ambitions was Ghost Hunter, and second was Firefighter!
I haven't even tried playing Legacy. After I'm done with Dynasty might try it.
It's good to know there is so many things left to do in this game!