This is the difference between equality and equivalence. It's not cross-platform to the extent that Dropbox is, though the fundamentals are portable to any platform that supports FUSE and thus GlusterFS. There's also no web UI, though there are already plenty of web UIs that can be slapped on top of any POSIX directory on a remote server. If you're the sort of Dropbox user who primarily stores and retrieves individual files through the web UI, this is not equivalent. For what I believe to be the more common use case of using Dropbox as a mountable location to/from which to sync files, though, it's quite equivalent. The fact that it's open source, deployable under the user's own control, and actually secure even makes it a better fit for many current Dropbox users' needs.
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 as a mountable location to/from which to sync files"
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.
Being non-mountable isn't a core good feature. It's a serious deficiency. If you want to store locally and sync so you can work offline, this is only one of about a hundred solutions. It just happens to be one that also allows you to use the file directly, that keeps your data secure and private (unlike Dropbox), etc.
Unfortunately, an offline syncable folder is the core feature of Dropbox, and anything that doesn't have it isn't, by itself, a Dropbox equivalent.
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. :-)
It sounds like you've had some bad experiences and I don't know which projects address those specific issues, but personally I've had good luck with Unison. Somebody else pointed me toward lsyncd which seems to take an interesting (inotify-based) approach which might meet your "in the background" requirement better, but I don't have any experience with it. I know there are lots of others, with varying levels of maturity and "intelligence" regarding things like conflicts and moves/renames, but it's such a crowded field that I don't claim to be an expert.
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.
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.