David Revoy

Experience :

Personnaly, I never really was a fan of this new behavior. That said , it's not a big deal and I can live with it easily . It's less ergonomic for my digital painting workflow within Mypaint<>Krita<>Gimp via the *.ora ( open raster files ) as it was before into 2.6. The behavior is just different and non consistent with my other softwares. I can't 'save' an *.ora anymore, I have to export. It's confusing because I use the 'save' button for it in my 2 other digital painting applications.   Just minor lost of time figuring my way on the Save/Export panel and brain gymnastic to remember I should behave differently now in Gimp.

Understanding :

It can be even surprising to hear me say that, but I like some aspect of this behavior and understand the choice of the Gimp development and respect it. New users aren't supposed to know all the files format, and guiding them to a single file format where all the working data are safe is a strong and right proposition of design. I teach digital art in school and workshops, and this user error trying to save all the layer of a magazine cover inside a *.jpg  is very common . Avoiding that is something good.

I also had a very good time with 2.8 when I contributed to re-texture a map of the open source game Nexuiz  , as well as when I had to perform webdesign task ( header, buttons, background ).
Its really cool to have a source file , and an export file with a path for the export saved in the source file. I could re-open very fast my sources *.xcf , and export to the texture *.tga , or the web *.jpg . It's a real boost of workflow into a project Gimp-centric. 

Ideal propositions :

So, the design is good, and make sens. In fact; the problem appear only for user like me using another source file in the workflow . Using Gimp into an ecosystem of software ? thats not part of the software design ; Gimp is made to be Gimp-centric , and this move prove of it quite well.
 Well, it could works if Gimp was as awesome as Mypaint and Krita for digital painting ; but thats not the case. Convince Mypaint and Krita to adopt the same design ( split Export and Save ) and work with the XCF file ? Sure... ultimate solution.  *.xcf  is an open file format, and I would be happy to have a unified workflow. But in reality.... No one have time for this :-)
Convincing Gimp developpers to add *.ora to the source files ? I guess they would reply 'no' because *.ora can't store for the moment Gimp 's vector path , text data , mask , etc... Too bad ; 2 years ago I was really believing each actor of libre graphic softwares to work to get open raster as a unique file format for storing the source.
So the situation is freezing ... and I guess things will not move for long now.

Workaround

My actual solution is the Save/Export Clean Plugin of Akanna . A little Python script to  download and put under your folder /home/<username>/.gimp-2.8/plug-ins and mark it as executable.
With the Edit > Keyboard shortcuts , I can attribute it Ctrl + S   , and I attribute Ctrl+Maj+S to 'Export to'.
Thats all , now things work as before for me , because I'm using mostly keyboard shortcut. Gimp doesn't warn me on closing because I didn't saved my file as *.xcf .

This workaround doesn't use the default 'overwrite' entry to map to Ctrl+S ( as proposed into this  article   ), because after a single 'overwrite' the option in the menu  disappear and let 'Export' replace it, and this one needs a new shortcut. I dislike this.
Other problem ; Gimp can't export to *.xcf , so, I can't with my shortcut do an export for a xcf files .