Maybe these have already been addressed, but here they are.
1) Early adopters may have stashes of millions of bitcoin. If widely adopted a few super rich could have a huge influence over the whole bitcoin economy. Imagine owning 5% of all USD in circulation(with no fed to inflate).
and once they sell their 5% of BTC the buyers of that 5% then have the control.. look at gold.. initially the US owned most of it in fort knox. now you can buy it at vending machines in dubai...
2) The supply is fixed at 21 million, but bitcoins themselves are not permanent. People die all the time without wills. People die with btc on a computer/flashdrive that no one knows about, they're gone. People don't make backups, their computer crashes, they're gone. You might think this is great, because now your btc are more valuable. But for a currency, it's not such a good thing, to have the supply eroding over time. Theoreticly, the amount of btc could eventually go to zero.
think about all the gold in peoples teeth fillings and in motherboards, and jewellry which are now in coffins/landfill underground, lost forever. eventually one day in many decades someone may have a fast enough computer to unhash a private key from its public key, and redeem the lost coins.. anythings possible.. calm down getting to zero is something your grand kids wont even have to worry about
3) Speed of transactions. If btc becomes very widely adopted, can it handle billions of transactions a day without speed becoming an issue?
this is probably the first valid worry that would appear in your lifetime. but there are multiple solutions to it. such as using paperwallets as bearer bond certificates. handing them across to trusted parties away from the network. also off the chain exchanges such as mtgox use to do, swapping mtgox codes between users instead of transactions on the blockchain. plus many other solutions to grow in the future
Don't get me wrong. I love bitcoin and the whole concept behind it. These are just questions of someone looking to learn more.