Moogle was disappointed by his new Arduino Uno. He thought it was pretty unrefined: rough edges, thin solder mask, poorly aligned headers. Arduino responded directly that this is an isolated incident. They will replace any less than perfect hardware. (A Moogle followup post)
Additionally, bongobat may have encountered a major bug in the circuit that switches between USB and external power in all Arduino versions. Pololu Robotics did some investigation and the result isn’t great:
Sparkfun has finished looking into the problem, and it sounds like they are not going to do anything about it. We have put a warning on our page for the Arduino, but there is not much more we can do, either.
After looking into the design, it seems to us that the circuit is just fundamentally flawed, and there is little reason to expect it to work well regardless of which MOSFET is used. The newer MOSFET makes the problem more evident, so it is easily verifiable in all kinds of tests. (These were not just contrived tests: on the one laptop we tried, we measured almost 200mA flowing into the USB port after powering it off. We verified that this occurred across the entire recommended operating range of 7-12V on VIN.) The new Arduino Uno looks like it will have the same basic problem, but we have not tried one yet.
While we do not know of any cases of damage to computers, 200mA is not a trivial amount of current, so we do not recommend leaving an externally-powered Arduino connected to your computer, especially while the computer is off.
Another frequent criticism is that routing on the new board is sloppy. What do you think of the new Arduino board? Have you received the hardware?
The first round of a new design is almost always dogged by some issues. The Uno was available just about everywhere the moment it dropped, it must have been a massive undertaking to set that up. Arduino is a popular piece of hardware with a very active community, someone’s going to find a few issues. And that’s a good thing. Open source projects encourage dialog between developers and users, the community can help identify problems and provide solutions. The developers can roll fixes into the next revision.