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).

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.

3) Speed of transactions. If btc becomes very widely adopted, can it handle billions of transactions a day without speed becoming an issue?

Don't get me wrong. I love bitcoin and the whole concept behind it. These are just questions of someone looking to learn more.