- Power regulation: why embed regulation in each board when it's not always needed? And what about regulation or step-up converters for battery-powered projects?
- Board voltage: There are good reasons to use 3.3V or 5V depending on applications, why are so few boards switchable?
- Programming interface: While USB (and its decoder chips) is ubiquitous, so (still) are serial and ethernet, and (increasingly) XBee. Why not make the programming/serial interface optional or external?
- Real estate: There is only so much room in the standard Arduino form factor. Why not use that space for useful circuitry that would be tricky to locate externally?
Here are a few boards comparably-priced to the Uno that may be more useful for real applications since they have the Arduino core but different circuits for power, communication, or extra capabilities:
- Wiseduino ($34 kit) has a DS1307 real time clock with backup battery, a 256Kbit EEPROM chip, header pins for the Adafruit XBee adapter, and 6-pin FTDI cable interface.
- Freakduino Chibi ($33 kit) has built-in 802.15.4 (XBee) radio and optional ($3!) battery voltage regulation so you can input from .7V to 6V.
- Arduino FIO ($25) has a MAX1555 LiPo charger, power regulation, and an XBee socket.