Bitcoin is still unproven. This site itself still warns that it's experimental beta software. There are fears that a bug will be found flooding the market with fake bitcoins
That's what I meant by "lack of trust".
Faking digital signatures? Has this ever happened with the algorithms used by bitcoin?
, or that a new, better online currency will be introduced
That wouldn't change what I said. Bitcoin would keep being better as money than gold. This new, better online currency would only be even better than that.
Don't confuse "being good as money" with "being good as an investment". Bitcoins are definitely more risky as investment than gold right now.
, or that someone with enormous computer resources will attack the network.
All this rich criminal would be able to do is to fraud transactions, by double-spending. That's not an "attack to the network" but rather an attack to some individuals who are unlucky enough to deal with the criminal. And, I really doubt that the eventual benefits one could get out of such criminal activity would be even close to the great cost of gathering more processing power than the entire network. Somebody with that much resources would probably earn more by just generating, actually.
All these might make everyone's Bitcoins worthless.
Worthless, I doubt it. At most, it would see a decrease in value if, for example, a better currency shows up or most governments worldwide declare a War on Bitcoins Exchangers. But that has nothing to do with bitcoin's qualities as money.
These reasons are why I, and a lot of people, don't put more than a small amount of net wealth into Bitcoin.
Again, you're confusing "being good as money" with "being a safe investment". I don't doubt gold is a safer investment right now than bitcoins.