Enjoyment of the sandbox is primarily a feature of players, IMO. Some people enjoy experimentation, and a lack of direction. Some people enjoy straight forward challenges, clear objectives to complete and a road map. Some people use their creativity to fill in details or connect them together to create a narrative and they enjoy that. Some people are far more literal, and do not derive as much enjoyment from immersing themselves in this shared narrative between themselves and the game.
What do you consider a sandbox?
I'm going to skip a clinical definition and just go straight to what I think makes a good sandbox. And that's interactions. You can call GTA games a sandbox, and on a lot of levels they are. There's seemingly a lot of interactions. With vehicle physics and AI behavior, meta systems like gang warfare and what not. But in truth the range of interactions are narrower than a game like DF, where some interactions spawn more interactions which spawn yet more interactions.
What do you consider too much?
It's more what I consider not enough. Again, a good sandbox to me has lots of interactions and details, which when combined together help the player create or reinforce an interesting narrative. What's not enough in a sandbox environment to me is that simply exists. Now, think about this. The term sandbox originally refers to a pile of sand with some edges to say where it stops, and that's it. As kids, with scoops and water and probably pee, we managed to create something out of nothing and invest it with our imaginations, making those tiny sand drifts into giant rolling dunes in our minds, that wad of sand formed by our sippy cup into a grand castle bastion.
So at its core, yeah, a sandbox in theory just needs to be a defined space with stuff for you to manipulate (interactions) so you can exercise your imagination on it. But a lot of us are young adults or adults now. As we grow older, our capacity for imagination and inventiveness diminishes to some extent. And so we need more than a blank slate to invest our imaginations into. Otherwise, we'd all be artists and game markers instead of media consumers. So what a sandbox also needs is some form of context. Some trait that contextualize what this micro reality is. Such as a star field (space!), an ocean (deep sea shenanigans!) a medieval world filled with monsters (DF!)
To put it plainly, sandboxes that are nothing but matter (sand, blocks, sims) and white space aren't enough. I don't personally play video games to fill in all the details myself, I play them to absorb all the hardwork the developers have done and synthesize it with my own efforts and imagination to arrive at the final product: my game experience. So in that sense, there is no "too much." There are just things I've yet to see and experience or experiment with.
Do you think that a game with elements of open-ended sandbox play needs to sacrifice short-term fun to do so?
Again, I say it mostly comes down to the players. I'll again take DF as an example (since we all know it and it can be fairly polarizing amongst its playerbase about what the real fun is.) I've heard some people say of DF "I hate how it takes a million hours of prep work to make something cool happen." For these players, it's about the destination and not necessarily the journey. (I say that in a non-pejorative sense.)
For others, it's about the narrative that is created along the path. Short-term fun, for them, is as much watching the development of a peasant into the Sheriff as it is about having that badass, impregnable, self-sustaining fortress.
So I guess my answer to the question is no, it doesn't. With the right amount of details and interaction, micro goals appear along the way to serve as short term fun.
The latter is indeed most annoying, when the sandbox aspect is little more than going from one part of the detailed, but annoyingly big map to the other.
So are you like, including Skyrim in this group?