Also, in most cases, it benefits you as a player to have the person sitting next to you know what you are drafting, so hiding the fact that you took a common is probably disadvantageous in general.
This is completely untrue. You must have the nicest meta in your country. If someone knows/guesses correctly I'm in a color and they don't see a card they want in the pack, they'll hate the crap out of that pack to screw me over. This has happened to everyone I know who drafts regularly, and frequently. Hell, last paper draft I did some guy just decided to hate on green in general, always taking green cards out of packs when he didn't want a card from the pack, including wasting his early picks on
Netcaster Spiders and the like, which really bolster a green deck. Worse, he still lost terribly, and had probably 7-8 playable green spells that he didn't play.
I'm not sure what illegal signals are either, unless you think picking cards out of a pack provides illegal signals?
I have two card shops I play at. At the first shop, which includes more skilled players, they actively talk about their first picks and any bomb rares that they are able to wrangle. In this group people generally stay out of other peoples colors if they can avoid it.
At the second shop, which includes a couple of people who know what they are doing and several newbies, they never talk the entire draft (which I prefer, and is the proper way to do it) but about half the people will just grab semi-good **** no matter what color it's in which makes it impossible to read proper signals. I had one guy in that group after the draft, he points to a pile of about 10-13 cards and states, "Those are the cards I wanted to draft. The rest of these I'm just gonna have to find some way to work them in."