16: tab are the evil
If anything has to be said about indentation capabilities of the Arduino IDE, I think CTRL+T is quite good, but an on-the-fly auto indentation feature like e.g. in MS Visual Studio would be awesome.
Especially because we'd see much more readable code in the forums
But hey, I won't be comparing VS with Arduino IDE..
17: relying on concatenation order of source files is total madness IMHO no, wait _concatenation_ of source files is total madness; #include directives and #define guards were invented for a purpose... if I had the power, I'd take that feature away from the IDE
OTOH, having more than a handful of tabs open makes switching between them a PITA... But again, it's an editor for "sketches", not for "projects". That must tell us something
18: serial monitor auto-opening would be most welcome.
19: direct hex upload from the IDE ("Upload compiled sketch" maybe ?) is totally useless IMHO given the target audience of Arduino.
20: the main problem I see with keywords.txt files is they add up regardless of the libraries actually used by the sketch.
Can't find the link right now, but there's at least one very popular ide mod around here. One of its marketing points, IIRC, was serial monitor auto-opening...