No, the biggest thing would have been stuff like swappable RF modules so I can keep the computer portion of the phone and get the RF module my carrier requires.
The biggest thing would have been support for external memory cards (microSD would have been great).
The biggest thing would have been ditching the 30 pin connector for a mini/micro USB connector.
The biggest thing would have been allowing some access to the file system.
The biggest thing would have been adding "guest" mode to the OS so you don't have to worry about access to your stuff when you hand the phone to someone else.
There are a lot of "biggest things" that would have made iPhone 5 truly a revolutionary step in the history of iPhone. Longer, thinner and blah, blah, blah doesn't, in my not-so-humble opinion qualify. Most people I talk to have said "not interested". As a developer I'll have to buy one. No real choice there. We'll see what the average consumer does.
The real question: Is the iPhone 5 enough of an improvement for the average person to ditch their 4S? I don't know. My rather limited sample came out on the side of "no".