GUI Editor To Preserve Markup
As much as possible, make it so that the GUI editor does not alter the format of the markup that has been created with the text editor.
There are definite advantages to editing in the GUI editor, but there are some things that are best done in markup. It is not unreasonable to want to switch back and forth as necessary.
There are many different markup formats that will produce the same end result, and when editing the markup you can choose the exact formatting that best meets your needs. The problem is that the GUI editor has no respect for your formatting and will change it to the way it likes it. I think this is bad.
At an absolute minimum, you should be able to switch from text to GUI mode, and back, and get exactly the same markup that you had before switching. (That is just a simple acceptance test.) To take it to the next level, if you only edit one line in the GUI editor, that should be the only line that changes in the markup.
Examples
Here are some examples of problems caused by the GUI editor reformatting your markup
Problem #1
User A creates a page using markup. User B edits the page in GUI mode and makes a single change. When looking at the Diff. to see what has changed, most of the changes were created by the GUI editor itself (especially on a long page) and are not material changes. This obscures the real changes.
Problem #2
Subpage links with alternate link text get changed to fully qualified links.
[:/SubPage:A SubPage]
Becomes:
[:FeatureRequests/GuiModePreservesMarkup/SubPage:A SubPage]
To some this might seem logical, since renaming a page currently does not bring the subpages along with it. However, I consider that to be a bug as well. If you have a page with subpages, renaming that page should automatically cause the newly named page to have all the same subpages that it used to. Unfortunately, you have to go through and manually rename every single subpage! Similarly, you should be able to keep your subpage links as relative, rather than fully qualified.
Note that if your subpage link was bare (just /SubPage) that the GUI editor does not convert that to FeatureRequests/GuiModePreservesMarkup/SubPage. So this is inconsistent behavior.
Problem #3
Probably the most annoying... The GUI editor removes line breaks following the [[BR]] macro. So this:
Two roads diverged in a yellow wood, [[BR]] And sorry I could not travel both [[BR]] And be one traveller, long I stood [[BR]] And looked down one as far as I could [[BR]] To where it bent in the undergrowth; [[BR]]
becomes:
Two roads diverged in a yellow wood, [[BR]] And sorry I could not travel both [[BR]] And be one traveller, long I stood [[BR]] And looked down one as far as I could [[BR]] To where it bent in the undergrowth; [[BR]]
This is just as annoying if you only use the GUI editor. It is perfectly valid to want single-spaced text on individual lines, and you should be able to view it that way.
Problem #4
GUI editor converts single linebreaks to spaces, in general. This is not as critical, but there ar still cases when you might want (non-paragraph) line breaks in your markup to make it more organized.
Problem #5
Until the GUI editor is flawless, there may still be cases where it unintentionally changes the markup in a way that affects how the page is rendered. This could be minimized if the GUI editor were careful to not change anything except where the user actually intended to change something. 'On the other hand, that may cause bugs to go undetected for longer, since they would be harder to run into.' Kind of a toss-up.
I'm sure there are others I've missed.