Risks

I have posted mostly on here about how positive bitcoin is, and how much potential is there.... I'm going to use this post to play devils advocate and try to destroy the network.   I'm 100% honest here..  What I will not do is make 'uneducated' arguments though.   There are certain arguments that are only arguments because the argue-er doesn't understand some basic fundamentals.   Those arguments are.....

-Bitcoin is not backed by anything (today, no currency is backed by anything)
-What if the internet goes down or the power goes out (the only thing with value here is food)
-I can't hold it in my hand, its not physical (of course you can)
-the government will regulate it (you can't hold a gun to algebra or the law of gravity)

Here are some valid arguments.
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In my opinion, software updates are the biggest threat to the Bitcoin economy.   A hard fork is an update that changes or removes some of the rules of the network.    A software update that dos not have75% of the consensus of the miners, or a software update that is execute poorly would absolutely destroy the network and everything it has built to date.

An update that splits the block chain into BTC1 and BTC2, leaves its users with 100% of their coins on each fork, at 50% of the value.

Luckily, anybody trying to push a software update to the network has their own best interest in mind.   Nobody with coins would try and push a malicious update.

However a botched software update could do serious damage.    Currently today, the bitcoin foundation would organize a hard fork.   Currently the network is small enough to easily organize, but in the future, nobody knows?

I'll just note here than ANYBODY can fork the bitcoin blockchain... but the value of your new blockchain will ONLY be based on the number of users that use it.    If the US Government wants to change the rules with their own blockchain, go for it.   Its actually encouraged..... but I doubt any miners, exchanges or users would use it.

I mentioned this in the previous post, but I'll say it again.  How would you run a software update to the law of gravity?
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51% and now 25% attacks are a small concern.   With each day, the growing mining difficulty makes it harder and more expensive to launch such attacks.    Again, nobody with money INVESTED in the system is going to work to destroy the system.... well perhaps a government might.    Each passing day makes the network more secure.
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A weakness in SHA256 or in Eliptic curve encryption could cause problems for the network, but not as bad as a botched update.

A weakness in the used encryption method would not happen overnight.... first, encryption methods are weakened.... so instead of taking 500 million years to break encryption, maybe it only takes 2 years.   The result is that the network has time to change the encryption method before its officially "broken"
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A better currency
  This is another great argument.

 What if somebody or something creates a BETTER internet currency.

An educated bitcoin user ENCOURAGES this.    Bitcoin is a challenge to the world.  You've seen the power of internet currency, now do it better.

To date, nobody has.  There are some 100 'alt coins' and a couple centralized systems like ripple.   None of them to date have anything on bitcoin.

Its kind of like saying to somebody, go home and code a better internet, or code a better email system.... it seems like it just can't happen.  If you did somehow manage to code a better cryptocurrency, bitcoin would just take your ideas and integrate it into the bitcoin system.

If it does happen, it will not be an overnight sensation.  This better currency will slowly win over users, and an educated user of bitcoin will split their value 50/50 amongst competing currencies.

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Unknowns.

There are any number of unknown potential problems out there.    However because the bitcoin blockchain is an infinitely secure database, the worst thing that can happen is that the network continues from the last known good block.  

If something breaks, the network rewinds to the last good block... gets consensus and moves on our way... however the damage done by the potential problem could take years to recuperate in market price.