Edit: Looks like I was wrong.
These things aren't dropbox replacements, they are reminders that dropbox is a unique product.
As for "never quite as good" that's just anti-open-source crap. It only works if you define a category by reference to a specific commercial alternative, and use that product's feature set as your checklist. That's no more valid than starting with the open-source alternative's feature set and using that checklist to find the commercial product inferior. It's marketing, not real life. If open source did exactly copy commercial products, then (a) technology wouldn't advance as much, (b) people would bitch about the lack of originality, and (c) the people who make the commercial products would sue. That's a totally unreasonable expectation.
Dropbox isn't "mountable". The files are stored locally and synced. This is MUCH different as you can work offline. Additionally, you get the performance characteristics of local access.
So this is equivalent in the same way say a flash drive, or Filezilla would be equivalent. It doesn't miss edge features of Dropbox, it misses most core good features.
Of the hundred solutions you mentioned, could you recommend a few that will automatically sync files in the background as they are changed with versioning/conflict resolution that don't choke on large files? I haven't found any which perform the task satisfactorily.
Edit: Tone is not intended to be antagonistic; I'd genuinely love to know what you use for this. :-)
My attitude is generally is that if you start with something that can be used live then it's trivial to use it for syncing instead, whereas something that's originally designed for syncing might never go the other way. Right now I just rsync to the remote directory, then from it, whenever I feel like. I could automate the process, or replace rsync with something more sophisticated, but my focus is mostly on the remote-infrastructure side because that's where I see a dearth of suitable alternatives.