1. The ~10 minute confirmation time
2. The reliance on set encryption protocols which could be broken in the future (EC and SHA256). For something to become as big as a world currency, it needs to be trusted that the technology would last hundreds of years.
3. The blockchain size.
4. The amount of resources that the client uses on a device.
5. Other scalability issues, such as the amount of transactions which can be handled per minute.
6. Irreversibility of transactions.
7. The extreme ease of the average person losing coins due one of many vectors of mistakes and hardware failures, or hackers.
8. The technical difficulty for the average user trying to even use it.
9. Bitcoin commerce and usage has mostly formed within centralized entities even though it is supposed to be decentralized.
10. There are still humans who ultimately control Bitcoin and who must be trusted by its users, even though the system is supposed to be decentralized.
11. Volatility - it is not an attractive store of value for someone who just wants to use it as a store of value or a currency and isn't into investing or speculating.
12. The present distribution of wealth.
13. The reliance on the internet in order to function.
14. Also, what happens if the internet is split apart due to government firewalls.
15. Governments, corporations, and hackers could ultimate control and restrict Bitcoin usage if they really wanted. And mass adoption will not be reached when users have to do it illegally, run tor, and run checks on every client they download to make sure it isn't a hacked client. Even tor traffic or bitcoin traffic over tor could be blocked too by smart routers.
16. Some of the worlds most powerful contries have already banned or restricted it.
** add more here ** - anyone else is welcome
TERA - All of these points/deficiencies can eventually be overcome with time and development, as you already know. Except for #14 through #16, which I agree with you... it may get crazier still in the next 2-3 years. #15 could happen if digital currencies becomes enough of a threat to gov't/banks throughout the world, but the U.S. and other democratic leaning countries already know that trying to stifle technological innovation, even in the finance industry, is just a really bad idea and wouldn't go over very well with the populous. It would backfire if they came down hard, because they loathe to take things away from people without cause.
Let's look back 15 years ago. Remember when you still had to purchase web browsing software? Remember when the free/open source software movement began? Remember the Java/J2EE vs. .NET wars? Remember when Linux came along to shake up IT in the Enterprise? I do... and I remember when folks were shouting at the top of their lungs that free, open source software would NEVER trump non-free, proprietary software. It would never work, they said, and would die a slow painful death. Well look how that all turned out.