...and here comes my own troubleshooting, too.
After I swapped things in and out of folders, in order to avoid third-party services and processes to run automatically at login, I placed again all bits and pieces in the right place one by one to check and observe which one might be causing the timewheel freeze at shutdown.
Found it, it's a popular anti-virus engine; quit it before shutdown, machine folds in a matter of seconds, way faster than with Mavericks, but if it's still on and running, it just drags for half a minute, or slightly more.
After having reproduced the flaw on another machine (as the one I'm having the problem with was indeed the only one in my pool to run it), I went onto the support file of the anti-virus. Other users are describing the same symptom, and there's a lot of activity in researching what causes the problem, which is the reason why I'm not quoting this application by name... it's good, it's solid, perfectly functional and greatly supported, they don't really deserve to be flamed for something that doesn't seem to be a fault in their own architecture.
Too bad, 10.10.1 dripped down this evening, I was secretly hoping it would have solved the issue, but it doesn't.
For the time being, I'll run the anti-virus engine selectively, on specific actions and instances, and quit it before shutdown if I'm in a hurry.
And, how comes, I don't have a problem in accepting a faulty behaviour when I know what causes it. I can keep it close to me, I can live with it, it won't bother me (to some extent, since performance isn't affected at all, just shutdown, which can be avoided by... just stopping the machine). It's actually not knowing what causes it, this or any other malfunction, to make it into a(nother) sleepless night into the quest for reasons.
By the way, selectively loading and unloading services and processes was the step I took after having re-installed Yosemite from the boot off the emergency partition (which, of course, didn't help at all).
Hope this can help someone else with yet another place where to poke in search of hints and (if luck helps) answers.
And now, even my last machine with Yosemite runs (and shutdowns) at blazing speeds. Lucky me? Not exactly.
Given on how many different machines Yosemite is providing me the same good performance, despite severe limitations in processing power, RAM and disk storage, i can't help but say that I see and appreciate a job well done when I see one. The very machines that now whirr merrily with 10.10.(1) were unmercifully sweating and trying to climb the hill with Mavericks and, much worse even in terms of overhead, Mountain Lion, and if you wanna shout curses to operating systems I believe there are a lot of better candidates to turn your anger to.
