Thanks for your input.
1. Firstly, I think text colour and highlighting is key. For all sorts of things, from errors, warnings, etc.
Also because web based languages can include PHP, Script and HTML all in the one page, I'm designing it to detect what the current language is and switch highlighting automatically.
2. Cut & paste, I dont think I can improve that at all. Allowing multiple items on a clipboard is out of scope for me right now.
3. Undo, actually one of my key features I'm working on is a full history system, so undo/redo is very easy.
4. Paren Matching - I hear you! In fact I was thinking of having a graphical overlay that draws a transparent line between the two selected parenthesis.
Declared variable font change - this is actually already been done in other IDEs, but I like it, and is part of the colouring/font work.
Code suggestions - I'm trying to think of a way to make basic suggestions such as semi colons, im thinking about using very transparent characters, that the users can click on to commit them.
As for arbitrary indenting - Code manipulation is something currently out of scope and something that will hopefully be looked at in the future.
Out of your suggestions, I really like the visual cues related to spaces and tabs, I hadn't thought of that one.
Thankyou for your suggestions... Has anyone else got any suggestions?