I travel quite a bit on business. This is what I carry on a plane:
Combining a couple of these items give you an effective "blackjack" and restraint devices.
Both of the bracelets in the first pic are made of 8' to 10' of 550 Lb test paracord. A 2' section combined with the SwissTech multi-tool from the keychain might make a functional defensive weapon if you use some imagination. As would the cable lock.
Pens are legal. So are soda cans, some knitting needles, chop sticks, keys, screwdrivers (check the TSA website for length limits) blunt scissors, and CD's.
With a little imagination, any can be used to harm an individual bent on taking a plane.
Personally, I think 9/11 ruined it for any would-be hijacker. Try to take over an aircraft today and most of the passengers would believe death is eminent. They'd do anything and everything to keep from being flown into a building. I think you'd have the support and cooperation of the entire flight when it comes to administering a thorough beat down on the first person to step out of line.
They're already PO'd at the "cattle car" treatment they received at check in, security and boarding. Then all the over-head bins are full. The guy next to you has some strange skin disease and won't shut up. A terrorist looking for an arse whupin' would be just the thing to break up the monotony. Especially if you have the middle seat near the rest room where the 350 Lb guy from row 6 just couldn't finish the 2 hour flight without stinking up the whole plane.
He might end up with some collateral damage.