tl;dr: Juliusz has improved button styles. Try them all on ee-flow-extra. Still some issues. One more hack day should do it!

Juliusz and May clarified a lot of stuff and he has a new patch that's closer.
* The quiet buttons get lighter on mouseover (hover) and darker on click (active)
* It adds a slight text shadow, other tweaks.

It's all on http://ee-flow-extra.instance-proxy.wmflabs.org/wiki/Special:UserLogin , together with the new living style guide, my button generator, and login forms using it, and latest Flow for comparison. The links are the same, repeated at the bottom.

Still problems:

* There's still some kind of glitch where on MediaWiki pages the background color doesn't darken on click.

* An <a href> with mw-ui-button mw-ui-quiet still gets a hyperlink underline on mouseover and click.

* The living style guide doesn't match what you see in MediaWiki, because Vector sets #bodyContent to 0.8em  (This is why the separate living style guide doesn't work for desktop!)

My comments:
Play with "A row of actions" in the living style guide[2] and the login form[3] as you read this.

* The text-shadow on colored quiet buttons makes them look fuzzy.

* In MediaWiki pages the form buttons are pretty small, just 12.8px.  The login buttons in Flow are 14px tall.

* We need a class for a right-aligned row of buttons, "mw-ui-button-row" ?

* Buttons need some spacing when in a row.  Flow gives its buttons a margin-left: 8px; , assuming they'll be in a right-aligned row; this could be applied to buttons that are children of the right-aligned mw-ui-button-row.

* The new styles implement white text on gray for the disabled colored (constructive, progressive, destructive) CTA buttons. This has been in the spec for months, and Flow never implemented it! I think it's confusing and jarring when you have two disabled buttons next to each other.

* In some thread Jared asked that the input focus indicator look like the mouseover state. I don't like that, and you can't find the input focus when it's on a quiet button because their mouseover state is a subtle change.

Responses to Steven:
On Mon, Jan 6, 2014 at 2:28 PM, Steven Walling <swalling@wikimedia.org> wrote:
- On login, when using a RTL language like Hebrew or Arabic, the right button border is messed up somehow. See http://ee-flow-extra.instance-proxy.wmflabs.org/wiki/Special:UserLogin?uselang=he Anyone else see this?
Seems OK now.
 
- The login form now has a mix of center-aligned and right-aligned elements.
Yes in 2014 Agora, buttons are right-aligned and the main CTA is on the right. (I added this guideline to the living style guide.)
 
- On login secondary "Join" link is placed to left hand of the main submit button. That makes no sense considering I read left-right in English.
Yup, that is an issue with quiet buttons.

Also, it needs to be an actual button, preferably a call to action, not a plain link. This is probably going to require a bit of a redesign to incorporate well. 
I think you're right. When should we use color and when should we use quiet buttons? In Flow when you type something into a reply box, you get
         Cancel  [Preview]  [Reply]
         quiet neutral - CTA neutral - CTA constructive
Only the main call to action has a color even though Cancel is destructive and [Preview] is progressive. If we let the button colors match their semantics then the buttons would appear
          Cancel  [Preview]  [Reply] 

In bug  57695 "Apply intuitive Agora styles to Flow buttons", Eduard Braun (is he on this list?) asked for the colorful look, saying all the gray buttons look alike. We have the option to be more colorful, just need to choose wisely.

 
- The main submit buttons seems a bit small. Probably should apply mw-ui-big or something.
See my comment above, the buttons are smaller than in Flow.  mw-ui-big is still around and I agree it would be nice to make the buttons stand out more.

I made these two changes to Login on ee-flow-extra[3].  When you're already logged in, the other button becomes [Create another account] and then the two buttons don't fit on one line, and because [Login] comes after it is pushed below the less important button.


Thanks for sharing on Labs S. You're the best. 

One other thing I notice: this style for mw.ui really begs use of placeholders instead of labels. Forms like Password reset etc. would look so much cleaner.
There were technical issues with placeholder-only in 2013 (Matt Flaschen had a patch), and there's the concern that if you return to a form or your browser auto-fills a form field with previous data, then you have no idea what the field is for unless you blank it out.

Links
As before ee-flow-extra labs instance has:

* Juliusz' patch updating mw-ui-button in core, https://gerrit.wikimedia.org/r/#/c/103494

* The patch that uses the new styles in Login, CreateAccount, and Special:PasswordReset (https://gerrit.wikimedia.org/r/#/c/104011/ )

* Latest Flow, try replying at <http://ee-flow-extra.instance-proxy.wmflabs.org/wiki/Talk:Sandbox> and compare its buttons.


* My left-hand nav hack that builds all the buttons, go to a non-special page like http://ee-flow-extra.instance-proxy.wmflabs.org/wiki/Main_Page , check mediawiki.ui, and then [Insert Agora buttons]

[2] http://ee-flow-extra.instance-proxy.wmflabs.org/w/resources/mediawiki.ui/docs/section-2.html
[3] http://ee-flow-extra.instance-proxy.wmflabs.org/wiki/Special:UserLogin
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=S Page  Features engineer