Share your thoughts.

Active careers.
Houseboats.
That's all right now I can think out of my head :/

The ability to pick up an entire room with all it's contents and put it down somewhere else, that TS4 introduced.

The Interaction to have your playable sim start and join in a three-convo. (I know "hang out" exist, but that's much limited I think).
More interactions for lifestates.
Witches having magic meter over reagents (crafting them is too time and motive consuming)

I have a 'no reagent mod'.
Here is the link: http://www.modthesims.info/download.php?t=299368
(In case you didn't have it)
Anyways, coming from TS4, I miss CAS somewhat, as well as the 'hidden' lots, and collections.

No reagent makes witches spell casting unbalance and op.

Honestly, TS4's build mode features in general, excluding the catalogue, cause it's so confusing to me. But there's a few things you can build in TS4 that take two clicks where in TS2 the same thing requires 4 cheat codes, 600 wasted simoleons, and like 15 different maneuvers to get stuff to go where you want it to go, and the simplest tutorial you can find for the thing you're trying to do takes like half an hour to follow. TS2 is very, very, very picky about elevation, which is understandable due to the way animations work, but it's also the reason 99% of my lots are completely flat corner to corner unless there's a beach. But that pickiness results in a lot of frustration when you just want a garage without an extension piece or a garage connected to the house... or part of the house on a foundation. Little things.

CASt - It would be nice not to have to fill my hard drive with recolors of clothes and objects and not need a lot of mods to edit wierd looking townies. Edit in CAS would be nice too.

Yeah, CASt deinitely, separate shoes and hats, placing lots far away from roads in-game, the skill system and gardening from TS3, Makin’ Magic from TS1 (alive gnomes!), upgrading objects, open areas like in TS4 with things to collect (but fully editable), swimmable ponds without any manipulations like in CS stories, the smooth terrain tool that works like in TS3, tombs, boats, scuba diving... There’s quite a lot I’d like from other iterations, but even without them TS2 has that something that makes me like it best

I want the entire expansions of Superstar and Makin Magic for TS2.

As RoxEllen said, CASt!!! (Seriously pissed me off so much that it was missing from TS4.) Totally agree about the "three-convo" thing too, by the way.
Also...
The ability to resize a room by grabbing a wall and simply dragging the room smaller or larger without changing the paint/wallpaper or windows/wall art, etc.
Horses. (And unicorns.)
The whole seeds/ingredients/planting system from TS3.
Sim-written books available for all sims in the hood to purchase.
Adults and elders can go to uni.
The whole "midlife crisis" thing.
Pets having wants.
The consignment shop.
Fairies.
The whole moon phases system.
Clipping coupons from the newspaper.
Pretty much the whole moodlets system.
Being able to feed toddlers real food in a high chair instead of generic mush.
Woohoo in the shower.
"Creativity" being split into like 10 different skills (or however many it was) so a sim who's good at painting isn't magically a master at playing the piano and vice versa.
The skill journal and collection journal.
Different "modes" while at work/school, such as "meet new people," "socialize friends," and "build skills."
Charisma skill being built through actual social interactions (eg experience) much like every single other skill.
Libraries/gyms increasing associated skill gains.
TS4's whole "controversial" gender-customization system.

There are probably more, but that's what I can think off right now.

SneakyWingPhoenix, one of my uploaded lots on here is a house boat. Sure if doesn't go anywhere but It gives the impression of being on the water.

I just want the diving and tomb exploring from TS3.

My computer cannot handle this all

@Annaminna, yeah, mine neither.

I agree build mode is confusing, I never built things in TS4, only furnished. I will agree though, that TS2 is VERY picky about placement.

I like the personality system, but I’d like if there were more opposing pairs than just 5 (for example flirty/unflirty, calm/hot-headed etc.), and some binary talents (like green thumb or virtuoso). However, I wouldn’t say that the overall personality system in TS2 is just restricted to personality points, aspirations have a huge part in defining personality of a sim.

Ohhh! Those would be nice! I've always wanted an honest/dishonest one myself.

I'd love to see a personality system combining TS2's personality points/aspirations with TS3's traits. More pets, especially horses. TS3 has a better inventory as well as storage containers. Open-world I'm kind of conflicted on but if it was toggleable sure. Separate shoes.

A sim being able to hold a toddler on their hip and use a cellphone with their other hand! (My mind was blown by that in TS3.)

The storage containers were totally boss too. I was a fan of the open world. Separate shoes/gloves/etc for the win.

Edited to add:
The thing where kids could do homework together
The little bake sale table kids could use to sell stuff they made in the toy oven
The toy oven making more than just muffins
All the goofy types of fish like "Black Goldfish" and "Alley Catfish"
How some foods would have different names depending on ingredeints. Like "Clownfish Sushi" and "Watermelon Cobbler."
Nectar making and wine cellars
Food having different quality based on cooking skill, ingredient quality, etc
Food having a little label on it saying who made it
"Kick it up a notch" and those herbs you could add for extra effects
The way chairs would "stick" to tables of all sorts so you could pick up the table and the chairs would move with it. (This is one my husband really misses, lol)
Being able to discard stupid wants!!!

To build pools literally everywhere (on second floor, ect.).

-Open World
-Story Progression
-Superstar
-Makin' Magic
-Traits (I love personality point. So I wish they could merge them to exist both.)
-Future changing. (Into the future)
-Quests. (Like in WA)

One thing that i really miss is the variety of reagents from MM and the witch duels, love them.

The supernatural and celebrity systems. Oh my goodness yes. I could do so much with that properly modded.

Edited to add: the imaginary friends, and the world adventures / travel system.

- More abilities to supernaturals. At the moment, lifestates are dull except witches/warlocks :/
- Lifetime rewards.
- Visit other households (I know there is a mod for that, but I would prefer vanilla version).
- Free recolor changing (grins my gear when I have to spend 15§ for a color change).
- Strollers.
- Cane.

I can not understand why Sims can not order their clothes online. The internet as we know it is quite old and the possibility to order clothes online must have been there at the time when the game was made. In the game it is possible to order food online or by phone. I can not speak for all countries but to order food online is something that only started growing few years ago. When the game was relesed we still have to get our food at the store. Most people still get the food there but that's an other story.