Enable multi-user editing
Allow multiple users, in a business to access the database at the same time so that multiple transactions can be entered or updated simultaneously, and do so with remote access capabilites like Terminal Server (Windows) or X-Term sessions (Linux/Unix) .
This is a feature in many commercial programs and this is exactly why many business users of such programs are not using GnuCash.
If GnuCash wants to secure donations from the business community beyond the home or individual operating as a business, this feature will make GnuCash extremely attractive to a market that routinely pays $3,000 to $5,000 for products lke Quickbooks Pro with $850-$1,000 per year support contracts.

Concurrent multi-user support has been part of our long-term goals for a while now:
http://wiki.gnucash.org/wiki/Roadmap#Database_and_QOF
Time to update the status of this feature request…
Note it still says “long-term”, which currently means several years ahead of us.
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Vladimir Bashkirtsev commented
I would agree that such "feature" will make us see GnuCash 3. Current code base clearly had no intention to become multi-user system. But I will agree with OP also: this feature is REQUIRED for GnuCash to move onto serious business scene.
My business is small but I'd like to have GnuCash running on my notebook and on my desktop in the office simultaneously. I found it relatively simple: as I am only user I can reload book from open DB over VPN and ignore warning that DB is already open. So it does the job but if I have someone else working on the same DB in my office it may throw things out of balance quickly and may be even corrupt DB.
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Admincstim (Core Developer, GnuCash) commented
I'm not opposed to this suggestion, but I'd just like to add that it will require an almost complete rewrite of gnucash. Subsequently, this rewrite will probably happen in another programming language (C++, Python, Whatever). And this rewrite will probably take 12-18 person-months until it reaches feature parity for the main business features.