Just for your entertainment, I'll tell you a little story about secondary fire.
This was the biggest sticking point in development, and for me, it was the biggest loss (other than not having more maps).
Basically, all weapons had two modes, a primary attack and a secondary attack.
Some of it was simple stuff, like the sniper scope secondary was a zoom.
Some of it was much funkier, like the Frying Pan, whose secondary fire was a block. If you blocked someone three times in a row they were temporarily stunned.
Another good example was the robo rat, the secondary fire was to teleport you to the location of the rat (instead of detonating it), giving you a high speed way to travel.
For the Katana, the secondary fire was to power up the attack, allowing you to run around with it held ready to devastate (assuming security didn't get you first!).
For the flare gun (which I think got cut?) the secondary fire was a bouncing flare, allowing you to bounce it round corners.
From memory, the glue gun had a similar bouncing secondary fire, allowing you to rebound it off a wall and into the path of someone chasing you.
I think the sleep sheep had an instant fire mode, and a secondary fire of being on a timer (delayed firing).
To be honest, I can't remember the rest of them, I'll try to dig up some of the old documents when I am back in the UK to see what the rest were. I had to try to forget them when it was cut as it was so damned painful to see them gone.
Now, despite these kind of mechanics being prevelant since HL1, and being used in L4D, guess what, it was deemed too complex for you all to grasp. A stunning waste of potential in my opinion!