Like the CODE keyboard, I will recommend the WASD Keyboards 87-key mechanical keyboard. The base cost is $145 USD, but it can get up to $185. Let's look at your requirements one by one.
- Must play nice with my 2012 MacBook Pro
This uses a standard USB connector with no special drivers. This will work with your MacBook.
- Must not slide around too easily, that's my pet peeve with keyboards
It weighs 2 pounds and has rubber feet on the bottom to prevent it from moving around. Even with the plastic feet up at the back to tilt the keyboard, it won't move around on its own.
- Must be affordable (ideally < $200, but much less would be better)
Depending on your choices, which I will get to at the bottom, this keyboard will run anywhere from $140-185 USD. It is a more expensive keyboard, but you do get what you pay for.
- Format should be US/QWERTY. Bonus points if I can rip the keycaps off and turn it onto Dvorak, if I ever get the ambition up to do so.
Since this is a mechanical keyboard, you can pop off the keys if you wish. However, the keys are only the same if they are on the same row. When you order, you get the opportunity to customize the key-caps. You can have both the QWERTY lettering and Dvorak lettering on the same keyboard.
There are hardware switches on the keyboard that allow you to switch the layout from QWERTY to Dvorak. You just have to unplug the keyboard to do it (more about this below).
- Bluetooth, I don't have very many spare USB ports
This does not come with Bluetooth. You are going to have to use a USB port.
- Have an option to have an OS X-style 'command' key symbol instead of the nasty Windows key symbol.
They offer the Windows, Mac, and Linux symbols for the keyboards.
- Not sound like hail hitting a tin roof, that makes me cringe when I type.
There are quieter switches available for this keyboard. Stay away from the green and blue switches. If you need help picking switches for your keyboard, check out Jeff Atwood's answer on Super User.
- Numeric pad: I hate those things and never use them anyway
This keyboard does not have a numeric pad.
- Portability: I don't have any issues using my MacBook keyboard when traveling.
This keyboard could be taken with you, but it might be best to leave it at home.
Hardware switches (on the underside of the keyboard) allow you to further customize your keyboard. Check out their documentation to see exactly what they can do.
You can choose how this keyboard will look and feel by changing the key-caps (the colors and lettering for each key), the type of switch, and add in O-rings to reduce noise from the keyboard. O-rings are like tiny rubber bands that go on the bottom of each key to reduce the noise of the key-cap bottoming out (known as the "clack"). O-rings cost $15 and $10 extra to have them install them for you.
The USB cable is removable. It is a standard USB-A to USB-micro cable.
You can completely change out the keys if you ever grow tired of the way it looks.
Some Cherry MX switches are more expensive than the others. Browns add $5 to the cost, Reds add $10, and Clears and Green add $15.
Unlike the CODE Keyboard, this is not back-lit. This can make it more difficult to type on in the dark.
The keyboard and the key-caps are quite thick. From the bottom of the keyboard to the top of the first row keys is about one inch. This can take some time to get used to if you are coming from a thin or laptop keyboard.