Here's what the expansions have to offer. I'll talk primarily about the Petting Zoo. To me, the game Coloretto isn't much of a game in and of itself. It's an interesting mechanic, but it's not a full game. Zooloretto is a game built off of that interesting mechanic. It's got some interesting ideas, but in my opinion the base game is little more than that mechanic. The thing that makes the game interesting is the money actions - the cool and interesting tactical things you can do that aren't "draw a tile or take a truck." In the base game there's not a lot of income. You start with $2, you pick up a buck on a truck now and again, people buy your animal once in a blue moon... it doesn't seem like enough to do much in the way of cool and interesting tactical money actions. The Petting Zoo allows you to get a little more income, and therefore players can afford to do more cool and interesting (even if oddly costed) money actions, making the game more of a game and less just like the Coloretto mechanic.

Also, in the base game there's often times when you really don't want the breeding animals, because they'll give you a baby you'd have to put in your barn (because the animal itself fills up your enclosure). That's kind of silly, considering the mating animals are pretty good, and you sort of want them most of the game, then at the end you're like "sweet, my mating animal came up... and now it's not only not-so-good for me anymore, but it's actually bad for me..." With the Petting Zoo expansion that doesn't really happen anymore (while it seems to happen a lot in the base rules).

So that's why I think the Petting Zoo is a must. The other expansions are simply more stuff to do with your money and I could take them or leave them. I like the extra enclosures, but they seem really strong- I haven't checked but it wouldn't surprise me if the winner of most games played with the Extra Enclosures expansion did buy one of the extra enclosures. The other expansions I only used once, the first time I played, and they were alright I guess. I had bought the Restaurant, which I thought was strong, as it's worth at least 2 points, maybe more, and I'm of the opinion that market booth thingies are really strong to begin with - worth 2-5 points each while other tiles are worth between 1 and 2 each.

So to sum up, I think the Petting Zoo is an essential expansion. The others are alright, but not as important. And the extra enclosures might be worth too many points for their cost.