I guess what you carry is a personal decision, and having said that, I will be the first to say I really don't care much what the next person thinks in regards to whether or not I am carrying too much. I think I said it before in the "snorkel or no snorkel" thread, the day that the next person's opinion becomes the major factor in my gear selection is the day I start looking for another pasttime.
So...I carry the following
Lift Bag (is rigged into my backplate, so it is always there anyway). A lift bag while a "must have" for tech diving or whatever, does not really serve as a signal device as they do not sit high enough out of the water to really be effective.
Storm whistle on harness
In a thigh pocket:
Yellow/Orange safety sausage (6')
OMS Dye Marker
Signal mirror (non-breakable lexan...available from REI...just keep it wrapped until use so it doesn't get scratched up to badly)
LED backup flashlight (always there day or night)
2 strobes
Finger spool w/ 150' line
Cylume stick
I guess in my mind that is enough, although if I were doing open ocean diving I would consider finding a place to put a container of fresh water. I don't carry all this stuff if for example we are doing some training in a quarry or other physically confined area, but I do if it is anywhere else. I have never been "lost", but have been on dives where the safety sausage was handy as a way to get the pickup boat over to us.
What I would LOVE to see is some sort of version of watch like the one made by Breitling for pilots. It has an incorporated emergency beacon that can be activated on the international emergency frequency. Granted it is not a cheap watch, but nothing by Breitling is...but certainly someone could probably make something similar for divers.