I wish I'd known how much FUN it was :) If I had I would have started playing with this stuff much sooner!

I wish I had known that every math class I would ever take I would use again, and more than once, and I sure as hell wish I had paid a bit more attention.

I wish I had a lot of hands on experience programming and using FPGAs. They are the new wave of embedded system development and I have only basic class experience with them.

I wish I had been taught how to use all of the modules of Microcontrollers in classes before my boss expected me to use them all.

I sure as hell wish that my university's electrical engineering department started design classes before senior year.

I think of loop as being the arduino's substutitue for main in C. So I declared some variables at the top of loop. Bad idea, they get re-initialized each time around the loop. Instead declare as globals before any subroutine. Not a big deal, but it took a few minutes to figure out what was happening.

I wished I'd had a clearer understanding of Fuse bits. I spent half my time early on terrified that I'd brick my mega32. Oh and ditto with regards to serial IO.

I wish I'd known how useful a board vise and a really good soldering iron would be. My Panavise Jr and Aoyue soldering station cost < $100 together, but they've paid that back many times over by making it a lot more pleasant to build things.

You don't absolutely need them, but they're much much better than helping hands and $15 cheapo soldering irons.

I wish i had arduinos and "make things talk" when i was a kid. I would have automated everything!

The CAN bus communication scheme and CANOpen.

I have used the Arduino a fair bit and I wish I had known how difficult serial IO was on arbitrary pins. I eventually settled on a 3rd party library but it took weeks to get to that point and even then it was not rock-solid reliable.

I'm with Kortuk. I wish we'd started simple design classes before final year of university, but specifically I wish we'd gotten to use an Arduino board in my classes!

We used a specially configured board designed at University of Wollongong based on an old Motorola chip, all programmed in assembler via a monitoring application. Pain in the arse!

Of course, I look up the course content for digital design now and they're all using Atmel AVR chips. sigh

I also wish I had more time to tinker with 'em, 'cause that's the best way to learn!

One thing I wish I knew better is analog circuit theory. 1s and 0s are easy...hook pin 1 of device A to pin 2 of device B.

A second-order band-pass Butterworth multiple-feedback filter...not so much. And then you have to worry about the characteristics of the op-amp, like whether the capacitive load on the output combined with the output impedance of the op-amp creates a pole which makes the output oscillate.

Or let's say you want to send an audio signal to headphones. You must carefully choose a DC-blocking cap, because it will combine with the impedance of the headphones to form a single-pole low-pass filter. Pick the wrong cap and you will neuter the bass frequencies.

Similar to the accepted answer, I wish I had known about Arduino's sooner. There were some designs I was looking at doing around the house. I have no problem designing my own boards with uC's, but I just didn't want to pay for the PCB costs at the time and then mounting SMT chips. So, those projects fell by the way side. However, now that I know about Arduino and all the shields for it, I'm considering them once again.

I wish I would have known that I could get free samples from a lot of online stores. Then I wouldn't have been forced to choose which project to get started on based on cost. And I wouldn't have been afraid to buy components that I wasn't 100% sure I could get working.

I wish I'd known just how easy it is to make a BSD programmer and get started. That would have saved a lot of time tool hunting. Of course I also wish I would have had the Bus Pirate sooner :)

And I still wish I knew where to get more chips cheaply (including delivery).