Earlier quoted context omitted.
As someone who follows gun laws, "compromise" often takes this form: Lawmakers: "We intend to outlaw all semi-automatic rifles with scary black stocks" Gun owners: "Please don't do that." Lawmakers: "Ok, let's compromise, we'll only outlaw scary rifles with collapsing stocks and vertical handgrips on the front." (time passes) Lawmakers: "There is a terrible loophole in our gun regulations..."
I've heard that the definition of "assault rifle" is mainly cosmetic; to what extent is that true?
Selective-fire means it's not just semi-automatic (one shot per trigger pull), it can also be set to fully auto (shoots as long as you pull the trigger) or burst (shoots a few rounds when you hold the trigger, then stops). These are essentially unavailable to the average civilian.
In popular usage, though, "assault rifle" means a short semi-automatic rifle with a plastic stock--anything that looks like a modern military rifle. Heck, I bet if you chopped down grandpa's bolt-action .30-06 deer rifle and put it in a black plastic stock, somebody would call it an assault rifle.
Personally, I think the AR-15 pattern rifle is a deeply boring and unattractive firearm. It's also become a symbol in a culture war: a terrifying "weapon of war" to some, a statement of rebellion and "owning the libs" to others. At the end of the day, though, there's just not a hell of a lot of difference between an AR-15 civilian rifle and, say, the Ruger Mini-14, except that the latter has a less scary wood stock.