Any changes have to survive the public. The sort of anti-counterfeiting measures that many countries use are not common in the US because the public has very strong resistance to changing the look (must be green!) and feel of our currency. One of the most effective changes is to make larger denomination notes physically larger (since matching the paper is the hardest thing for most counterfeiters, they used to take $1 notes, remove the printing and then print a $20 or $50 on top of the paper).

50 cent and $1 coins are not commonly used in the US due to decisions of the companies that made coin acceptors (the part of the vending machine that takes, counts and validates coins) back in the 1950s and 1960s. Basically, they did not want to hand out 45 cents of change from a 50 cent piece (nor 95 cents change from a dollar coin) so they deliberately chose not to support those coins. Consequently, they stopped being used by many in the public as they were "inconvenient." As those coins became less common, the companies that made cash register drawers chose not to make bins for coins that weren't in common use, so there became no easy way for businesses to take those coins.

The low usage of $2 notes is also reflected in cash register drawer designs - there is no place for those notes, so most cashiers toss them under the drawer along with checks and large ($50 & $100 notes), so the circulation of $2 is very low. I personally like to get stacks of them and spend them instead of carrying $20 notes around. Some places, like at the Renaissance Fair, hand them back as change to the next guy (there is no slot in the till for them) so the next guy gets something they've rarely seen and they get thrilled.