The main problem is unstability of rates.also bitcoin is not available in most countries. so people can't easily buy and sell there
i doubt that it is that big of a problem to be honest because the fact that bitcoin is not stable isnt bad at all, in fact it allows you to earn a lot of money out of trading and attracts new people
my biggest problem with bitcoins is the fact that confirmations sometimes might be really slow as it might take an hour for bitcoin transaction to be confirmed sometimes
Delay in confirmation is also not a major problem I guess, as only sometimes it gets delay, majority of times its done in maximum 20 minutes and I am very much satisfied with it.