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.
Echoing some other responses...
1) I think the upper-bound is Satoshi with a *max* estimated 1 million coins. Not likely the other early folks have more than a couple hundred thousand. That is indeed huge influence, but an order of magnitude less than you're worrying about. Those balances are likely to disperse a good deal as bitcoin value increases.
2) Divisibility is really important. In a currency that's a pain to divide and transport, yeah, diminishing supply becomes problematic. But with bitcoin's divisibility, the world could function fluidly on 1 million coins if necessary (and divisibility can theoretically be increased by orders of magnitude too).
3) This rightly gets a lot of attention, with a lot of proposed solutions. Off chain transactions, various remedies to 0-confirms, Open Transactions, etc... Solutions will arise as the need becomes paramount.