Re: History and all
@keithpeter - "I think that I remember a spat about notification libraries as being one of the reasons for the decision to build Unity as an alternative shell as well as concerns around basic usability."
The notifications library issue came about later, as part of Freedesktop.org discussions (an organisation which was dedicated to promoting standards and interoperability between Linux desktops). KDE and Unity (Ubuntu) worked on standards for notifications and associated widgets, and Ubuntu produced a library which followed that standard. Gnome meanwhile refused to take part and then said they wanted nothing to do with it because "they hadn't been consulted" (because they had refused to discuss it - some serious circular reasoning there).
The head of KDE was quite pissed off with attitude displayed by Gnome, and gave his summary of the situation here: http://aseigo.blogspot.ca/2011/03/collaborations-demise.html Long story short - KDE felt that Gnome had a major problem with NIH syndrome and didn't play well with others. I'll speculate that some of that attitude may have been sparked by jealousy over KDE seeming to be the ones who came up with most of the good ideas. The KDE and Unity teams on the other hand focused on technical issues, avoided politics, and seemed to get along well enough.
@keithpeter - "It is worth mentioning that Canonical Design actually carried out and published the results of usability studies, albeit with basic tasks and subjects new to Ubuntu."
Yes, although I can't recall the name of the project. They went out and hired some professional UI usability consultants who produced a report which was then published and available to be used by anyone. Nobody else in the Linux world was spending that sort of money on usability by the average sort of person (as opposed to the sort who reads El Reg).
The project was called Ayatana. There are various wikis and Launchpad projects associated with it, but I don't know where the actual study is.
@keithpeter - "There appears to be very little usability research published for gnome shell that I can find;"
The Gnome developers wanted nothing to do with airy-fairy designers and people like that. Their design process consisted of someone writing some C code and asking one of their friends if they liked it. Conflicts were resolved based on who was friends with who at Red Hat.
The end result shows it. Start up Unity and there are familiar looking icons right there for you to click on so you can get started. Start up Gnome 3 and you have an almost completely blank screen and no clue on what to do next. It takes two or three times as many mouse clicks to do basic tasks in Gnome 3 as it does in Unity.
The most common tasks in Unity have keyboard short cuts (about three dozen of them) and they have a nice cheat sheet to explain them that pops up if you hold down the flag key. Gnome 3 is just starting to get around to implementing more than a handful, and if you want to know what they are you'll need some good Google skills to find the Gnome wiki.
Hover the mouse over something on Unity, and a tool tip will pop up telling you what it is. With Gnome 3 - well let's just hope you have a good memory for obscure icons.
I could go on like this for quite a while. Unity works on the principle of giving you obvious things to do up front, and making the important stuff large enough for you to see what is going one, and having explanations right there. With Gnome 3 - let's hope you like microscopic grey on black icons because you're going to have to memorise a lot of them.