The main reason why I'm waiting forward TS4 is that it promise to be an easier game than the previous one . I haven't played TS3 since more than 8 months (except for building 1 or 2 houses or testing store contents) because this game is too complicated now. We need tons of CC and mods to make it playable. There are too many bugs and issues and I did spend too much time on it in order to play my families. Moreover, my computer is getting too old to handle all the EP's and it's a nightmare to use boats or doing scuba diving in Island Paradiso.
Another thing I love in TS4 is the gallery. You want to buy a new house or even a room to your sim? just open the gallery, choose one and replace your previous house. In TS3, we have to close the game, open the launcher (which is sooooo slow), open the exchange, download the house, check for CC, if there was any CC deleting it using a custom program, then install the house, return to the game, delete the old house and replace it on the lot. You want new sims in your neighborood as well? Just open the gallery, select them and they'll be in your game walking around as if they really live in your town. It's pretty easy and faster for players who don't like to spend too much time on a game, like me.
The build mode is nice too. I' have never been a builder in the sims serie. I don't like to build houses and I'm lost when i've to use cheats or terrain tools. I know most players are unhappy because the terrain tools are basics and all lots are flat in TS4.. But I'm happy of it because it'll be easier to build houses. And no more issues caused by "dangerous" cheats like constrainfloorelevationtrue or else..
Another thing I find really crappy in TS3 are the sims themselves. I enjoyed TS3 when I played it. But I have never been fond of the sims. They're too "robotic", doing actions like robots with only idles behaviours to make them alive. I'm waiting a lot of the new emotions system. Finally, we'll have sims who behave like humans, being sad, happy, depressed...
The things I'll regret in TS3 are more linked to the EPs: seasons, horses, swimming in the sea and other interesting features. But it's more likely that we'll have it in TS4 too in the next few years.
So, even without SP or family tree or CASt, TS4 seems very promising to me.