In
this page they are talking about integration with different operating systems.. namely Windows and OS X.
"Personally I think a unified cross platform UI results in applications that at best feel foreign everywhere, and at worst don’t even feel like real applications. Making XUL appear truly native means we have to do some extra work when new operating systems are released, like Vista and Leopard, but I think this work clearly results in a superior user experience."
Before I start I want to mention that one can talk about two kinds of unified. Unified as in the toolbar is unified with the titlebar and unified as in the application looks the same on different operating systems. It can get a bit confusing but bear with me. I've avoided talking specifically naming a unified toolbar here. So when it says unified here it means the latter (the same look across applications\platforms).
One problem I had with the Firefox interface besides how foreign it looks on OS X is how foreign it functions. The menus for example don't look or act like a normal OS X menu they feel like half baked windows menus... the layout is a bit off, and they don't respect the transparency or colors of a theme. They can take care of the look mostly with themes but what about the feel? Stuff that's more than just skin deep.
Another thing is themes mostly support Safari or Camino if the person making the theme happens to use it. I like a themes supported browser myself that doesn't require me to apply a skin separately. So my favorite browsers are either Safari or sometimes Camino.
One other thing.... I hate how iTunes is insisting on using it's own resources for the window background\titlebar\status bar. Someone on macthemes said that it's because it's cross platform. But I'm like well so is Safari and it uses native resources for those things in OS X and hard coded resources in Windows. One reason why I hate the way iTunes is is because it's making some people decide like Mozilla decide to follow its example and hardcode those things into the UI so in Leopard it doesn't do what windows there should do when they go out of focus. Some other applications like Path finder did this and I really wish they didn't.
I don't think it's so important for the Application to look the same on 10.4 and 10.5. They should follow their own advice when they said that it's not good to have a unified look across platforms and now apply that across OS revisions. On 10.4 it should look like 10.4.. on 10.5 it should look like 10.5. When you change a theme it should look like that theme. Path Finder did the same thing recently for the same reason hard coding the toolbar BG into the window and it looks out of place when I change themes. I think developers like them should approach it like... "Let's try to hard code as little as possible"... especially when it comes to Window backgrounds.
Apple is touting that the windows turn white when out of focus as a feature... but they fail to mention, "Except for these one or two applications that follow their own rules". =P I guess there is mostly the system look and the iLife look but I don't see why iLife should have to duplicate those resources when they look mostly the same only they don't have an out of focus state. Maybe Apple's not really serious about unifying the look of their OS.
Also on Vista I think they should make it look more like Explorer (with a glass toolbar) than the WMP toolbar look they have going but I don't know how technically feasible that is... I did read an article about why they think they can't do it.
One last thing... on Linux they'd have to make different themes like for tango, oxygen, etc.
In short...
Unified Toolbar = Good (depending on theme)
Unified look compared to other applications on the same system = Good
Unified look across different platforms = Bad
Unified look across OS revisions = Bad if the systems look was changed between revisions.
Of course some people don't care as long as it looks good to them though.