I've been playing with the new and old firmware of the Airport Express 802.11n and I found a few but rather annoying bugs last night:
1) Airport Utility doesn't find Airport Express 802.11n
With firmware 7.3.x this seems directly related to the base station name. I had my base station named "Airport Express 2", and after that I had to reconnect it to ethernet very often to access the damn thing. But after removing the spaces in the name (AirportExpress2) Airport Utility found it every time. With 7.4.x that seems to be fixed, except that restarting sometimes goes faster with a "spaceless" base station name.
2) Airport Express 802.11n can't connect to 5GHz network.
That is a bug in firmware 7.4.0 / 7.4.1 - it just won't connect to another Airport base station regardless what channel is selected manually or automatically. This is the case in "Join a wireless network" mode and "Extend wireless network" mode. Only workaround for this is downgrading to 7.3.2 (select base station in Airport Utility, and then select Upload Firmware in menu "Base station")
2a) firmware 7.3.x makes very slow connections in "Join a wireless network" mode. Workaround can be, if you have a Airport Express or Extreme 802.11n as a main base station, to change the configuration to "Extend wireless network".
3) Skips and gaps in AirTunes playback on Airport Express 802.11n
This seems to be a bug in the firmware (7.3.x and 7.4.1). Although it appears more often when using
Airfoil. There are frequent gaps and skips on the AirTunes output. Even when the unit is placed very near the base station. When using both Computer speakers and Airport Express audio output with iTunes (or AirFoil) one can easily notice a large and rather quick changing phase shift between Computer output and Airport output. The older Airport Express 802.11g didn't do that, and almost never has gaps or skips in playback when placed within reasonable distance from the base station.