Some more details. This is the latest output from "strace" as running against an existing gnucash process, just before hitting the "File->Quit" option:
"""
[pid 13334] 17:18:16 open("/home/username/.gnucash/expressions-2.0", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 14
[pid 13334] 17:18:16 write(14, "", 0)   = 0
[pid 13334] 17:18:16 close(14)          = 0
[pid 13334] 17:18:16 open("/home/username/.gnucash/stylesheets-2.0", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 14
[pid 13334] 17:18:16 fcntl64(14, F_GETFL) = 0x8001 (flags O_WRONLY|O_LARGEFILE)
[pid 13334] 17:18:16 _llseek(14, 0, [0], SEEK_CUR) = 0
[pid 13334] 17:18:16 fstat64(14, {st_mode=S_IFREG|0664, st_size=0, ...}) = 0
[pid 13334] 17:18:16 write(14, "(let ((template (gnc:html-style-sheet-template-find \"Plain\")))\n  (if template \n    (let ((options ((gnc:html-style-sheet-template-options-generator template)))) \n\n; Section: Tables\n\n\n; Section: General\n\n\n; Section: Fonts\n\n(let ((option (gnc:lookup-option options\n                                 \"Fonts\"\n                                 \"Text cell\")))\n  ((lambda (option) (if option ((gnc:option-setter option) \"Ubuntu 9\"))) option))\n\n(let ((option (gnc:lookup-option options\n                                 \"Fonts\"\n                                 \"Number header\")))\n  ((lambda (option) (if option ((gnc:option-setter option) \"Ubuntu 9\"))) option))\n\n(let ((option (gnc:lookup-option options\n                                 \"Fonts\"\n                                 \"Account link\")))\n  ((lambda (option) (if option ((gnc:option-setter option) \"Ubuntu Italic 9\"))) option))\n\n(let ((option (gnc:lookup-option options\n                                 \"Fonts\"\n                                 \"Number cell\")))\n  ((lambda (optio"..., 3954) = 3954
[pid 13334] 17:18:16 close(14)          = 0
[pid 13334] 17:18:16 close(3)           = 0
[pid 13334] 17:18:16 munmap(0xb7430000, 4096) = 0
[pid 13334] 17:18:16 --- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_ACCERR, si_addr=0x9aa0740} ---
[pid 13336] 17:18:16 <... poll resumed> ) = ? <unavailable>
[pid 13348] 17:18:47 +++ killed by SIGSEGV (core dumped) +++
[pid 13336] 17:18:47 +++ killed by SIGSEGV (core dumped) +++
[pid 13335] 17:18:47 +++ killed by SIGSEGV (core dumped) +++
17:18:47 +++ killed by SIGSEGV (core dumped) +++
"""

FD number 3 happened to be:
"""
COMMAND   PID     USER   FD   TYPE     DEVICE SIZE/OFF    NODE NAME
gnucash 13334 username    3u   REG        8,3      176 2490542 /tmp/gnucash.trace
"""
And the referred file contents were as follows after the crash on shutdown:
"""
$ cat /tmp/gnucash.trace
* 17:12:12  WARN <gnc.backend.dbi> [gnc_module_init_backend_dbi()] No DBD drivers found
* 17:12:33  WARN <gnc.backend.dbi> [gnc_module_init_backend_dbi()] No DBD drivers found
"""

That seems to match quite well the crash on dbi_shutdown_r reported earlier on, so despite only logging a WARNING, the root cause for the crash on shutdown may lay there.

So I took one additional step: trying to provide gnucash with some valid libDBI implementation for DB backends. Prior to that, I only had the following installed regarding DBI:
"""
$ dpkg --get-selections | grep dbi
libdbi-perl					install
libdbi1:i386					install
"""

And then I just installed the following:
"""
$ sudo apt-get install libdbd-mysql
"""

After doing so, I can now happily start and quit gnucash with no further crashes, SIGSEGVs or any other woes of the same kind.

So recapping what's on this Bug, it looks like gnucash as shipped by Ubuntu 14.10 needs the following two additional packages to work fine and not crash at all:
"""
- python-gnucash: helps with avoiding gnucash to barf errors on startup, but causes gnucash to crash when quitting
- libdbd-mysql (or probably any other SQL-related DBI library): on top of the above, it makes gnucash avoid a SIGSEGV on "dbi_shutdown_r" on shutdown, and therefore quit cleanly


Hope this helps others and the package maintainer as well.