Well the only thing I can see is that EE.SE is unfriendly to questions that are not well thought out.
Let's face it: Everyone here expects the OP to have read the relevant technical documentation, which in itself is quite complicated and difficult, to be able to provide relevant information and there are numerous questions where users are simply chanting "SCHEMATIC, SCHEMATIC" and OP fails to deliver one. There's also the distributor side: From what I've seen typical distributors of Arduino community are different from typical distributors of EE community and often Arduino community distributors simply don't provide easy access to relevant information about their products. I remember question in which OP asked about using stepper motor with Arduino. He provided link to distributors webpage and it took some effort just to find the pinout of the motor. Some basic information like power ratings or normal coil resistance simply wasn't there.
Another thing that's often assumed is that OP is actually in control of circuits which he uses (and let's face it: Arduino users wouldn't be using all that much Arduino if they could easily make their own PCBs with needed components) as well as software (and from what I've seen, standard Arduino procedure is to use already available libraries even for "simplest" tasks).
There's also the measurement instrument problem which I think many Arduino users face. Here it's more or less assumed that everyone has at least a good multimeter. Oscilloscope is also considered a basic tool of electrical engineering, but that's something that most Arduino users won't have access to.
Another thing would be familiarity to the ecosystem: I've seen many Arduino users expecting us to actually know names of each shield and what components it has and so on. On the other hand, more or less each board a "typical" user here makes would be a custom board. There's the ever-present hatred for Fritzing diagrams here (I understand why they aren't all that useful, but many Arduino users won't), which seem to be the default way of presenting information for the Arduino community. Learning how to use even the simplest EDA tools is quite complicated and probably isn't what an Arduino user would be interested in, at least initially.
Another thing I've noticed in the Arduino community is what I'd call cargocultism. Very large number of tutorials don't actually explain what's happening in any detail. They simply list the spell ingredients and magical words to be written into the IDE. This may seem a bit rude to that community, but I understand that most of its members don't actually want to know details. They just want to get whatever it is they're building to work.
So if there's an Arduino web-site that will cater to users who have the habits I've listed, then I guess such community would be beneficial to them, since they'd have a "friendly" environment in which they could grow.
One thing I fear is that most problems I've noticed with Arduino questions here are also typical problems of what we call "bad questions". This is also related with what we expect to be the "due diligence" when posting a question. Something obvious to an EE wouldn't be as obvious to Arduino user.
I know that this wasn't part of the question, but I'd like to write a little bit the other side: What would be the downsides of having an Arduinio site: Well first, we'd be separating communities and doing so a bit artificially. This could might the "grumpy experts" to down and close-vote Arduino questions here. Another side-effect would be the encouraging spread of bad practices that are currently common in Arduino community. For example I've seen many posts that show something as simple as using interrupts instead of busy-waits as a revelation.
There's also the fact that sooner or later Arduino user is going to have to go outside of the Aruduino comfort-zone. Here we'll be able to provide the needed guidance, which may or may not be available on Arduino-specific site.
Finally there's also the fact that we have both Ubuntu and Unix sites. I understand that some people like their GNU/Linux with "training wheels" and I see that similar situation exists with respect to the Arduino and EE.SE.