Slightly different autoreset circuit.
Because of the different autoreset circuit on Uno, the hack to crater it into not functioning (disabling autoreset) is different.
If you look at the Uno Rev2 and Rev3 schematic there is now a pulldown on the signal
line from the 8/16u part going into the cap that goes to the reset line on the AVR.
The signal line from the FTDI chip (or the DTR connector pin on the pro) going to the cap does not have a pull down on it.
If I remember correctly, I believe this was added after it was shown that because of the simplistic circuit
that under certain conditions of using the RESET line, severe ringing with overshoot occurred
on the AVR reset line and bricked the AVR by tricking it into thinking it was going into high voltage programming mode.
I think the arduino 2009 had the best circuit. If the RESET-EN jumper is desoldered or c13 is removed and then r2 is
in place then RTS can be used instead of DTR and all this mess goes away including the ringing issue.
On the pro you have the option to simply re-wire your serial adapter to use RTS and feed that into the DTR pin on the connector
to avoid the need to disable auto reset.
--- bill