For example, there is only ONE New York city, and everyone regardless of their religion knows this and they would probably know what you were talking about if you said you were going to New York city.
Clearly, such is not the case for conceptions of heaven and hell.
Therefore, assuming that heaven is supposed to be a place that actually exists, it stands to reason that everyone regardless of their religion should have the same idea of what and where heaven is, and they should also all be thinking of the same place when someone refers to "heaven."
It seems to me that most people believe heaven and hell to be literally otherworldly in at least one sense.
Many think of them as purely symbolic concepts, environments with non-physical existence, or actual physical places that nonetheless are not part of this sphere that we call Earth.
(Comparatively) very few people seem to actually expect to find either realm on the surface of this planet.
I myself am a Christian for what it's worth, but if, for example, Christians have a different idea of what and where heaven is than do people of other faiths, then this basically says that heaven is a place that exists only as a matter of people's religious beliefs (since it can be different based on your beliefs), and not as an actual place.
It seems to me that many people would indeed reach such a conclusion. I don't think that is a problem at all, personally.
If, on the other hand it is supposed to be an actual existing place, then everyone regardless of their religion would and should think of it as the same place in the same location and should all consider themselves to be going to that place when they say that they are going to heaven. Of course the same argument could be made about hell, but my point still stands.
I think a strong can be made that such a conclusion is actually unproper, bad religious practice.