Trying to understand decoherence... how would macroscopic objects behave without decoherence? I've read it said that decoherence is the reason we don't observe macroscopic superpositions. I find this very confusing... what exactly does it mean to observe a macroscopic superposition? Doesn't the superposition go away upon observation?
What types of behavior would macroscopic objects exhibit if decoherence wasn't happening... what types of things would we actually see?
 A: There are two things that mustn't be mixed up in this context.
Say your system is in a completely random quantum state. You can freely change basis to make this state a superposition or not. To make a statement like "this is a superposition state", you need to specify which basis you're working in.
A quantum system evolves while interacting with its environment, and this process will select the basis for you. In most cases, since most external forces depend on position, you'll be working with position eigenstates.
This process is called decoherence: in a specific basis, base vectors lose coherence (similar meaning as in optics: they no longer interfere with each other).
This is, as far as we know, unrelated to the measurement problem. In a nutshell:

*

*Decoherence tells you in which basis you're working, due to the influence of the environment.

*Collapse of the wave function tells you which eigenstate is observed in that basis after a measurement.

What happens during collapse is actually unknown. There are some theories, some of them quite serious and interesting,  but as far as I know none of them has led to anything that can be checked experimentally.
Back to your question. Disabling decoherence boils down to disabling the influence of the environment, so asking what we would see in this situation is in fact self-contradictory. We wouldn't see anything because no interaction means no observation.
A: When we say we "observe" that an object has a particular quantum state, we do not typically imply that we do so with a single measurement. Instead, we prepare the desired state many times, and make many measurements, so that we have a probability distribution of observed eigenstates. We can take the probabilities we get from our series of measurements and convert them, up to an overall phase, into coefficients in the original state wavefunction.
A question such as "What would we see if decoherence wasn't happening?" is moot, because seeing an object requires that photons from the environment constantly interact with that object, which cause decoherence.
