* Why not to use Ctrl+PgUp/PgDown to switch among tabs?
  It's what gnome-terminal/epiphany/firefox do... What about Ctrl+Tab?

Ctrl+Tab is used to move the focus and it's fundamental for
accessibility, so it isn't available.
Ctrl+PgUp/PgDown is used by GtkTextView (the text display widget of
gtk+ which gedit uses) to move to the start/end of the current
paragraph. Use Ctrl+Alt+PgUp/PgDown to switch tabs.
Also note that this whole behavior is implemented by default by
GtkNotebook so go bug someone else ;)

* Gedit is very slow and/or crashes when opening files with very
long lines, can you fix it?
When designing GtkTextView (the text display widget of gtk+ which gedit
uses) the developers had to make a design decision trading off bad
performance and memory use on corner cases (like the very long lines)
in change of better performance in search operations and full support
for utf8 text.
This is a known limitation of the widget and cannot be fixed. (bug XXXX)
On top of that pango seems to use a lot of cpu drawing such long lines,
this /may/ be fixable, but it isn't easy... feel free to give it a try.
Crashes with long lines are usually due to out-of-memory conditions,
but if that's not the case then we would like to know about it.

* I want to indent a selection using the tab key.
gedit has an 'indent' plugin which allows to indent/unindent a selection.
The plugin uses the Ctrl+(Shift)+T shortcut.
Many people do not like that shortcut and would prefer to simply use Tab and
Shift+Tab. Such request was turned down for a long time because we were
concerned about the use of the Shift+tab combination which is also used to
move the focus and thus it is important for accessibility.
After a discussion with a11y people it was decided that only using Ctrl+Tab
for focus handling is enough, so the requested change will be implemented in
future versions of GtkSourceView (see bug XXXX)

* I want an Single Document Interface
Go Away.

* Really I Want SDI!
Die.

* I want to hide the tabs and just use the sidepane document list
You probably suffer some mental disease. If you cannot afford consulting a
specialist, you may consider this (XXX link) plugin from the gedit-plugin
package which allows hiding the tabs.

* gedit supports python plugins, but I prefer to write in
Ruby/scheme/perl/C#/modula-2/whatever.
Currently gedit embeds python in the core and we are pretty happy with it:
python is a really nice scripting language which allows fast developement 
and it has well tested gtk bindings.
If performance is critical, C plugins are still possible.
The power of GObject would allow to bind the gedit plugins system in many
other languages (provided they have gtk and gtksourceview bindngs) and if
you really want to pursue this task, we would probably accept a patch to
dynamically use other plugin loaders. However keep in mind that running
plugins in many languages doesn't come for free, since it means having to
load more than one interpreter into memory.