(1) Regulators and (2) technological problems.
The rise of the cryptocurrencies goes in line with the megatrend called 'empowerment of individual'. People get richer, they feel stronger and prefer less control over them, especially over their money.
After Bitcoin got sufficient visibility, it grows fast. On the one hand, there are entities that really use it for payments or value storage. On the other hand, there are lots of speculative folks who simply hope for quick money and throw their cash into a sort of financial pyramid. Nothing comes out of nothing in economics - if someone gains, someone loses. In other words, Bitcoin holders get richer because other people put money into their pockets.
Bitcoin is the flagship for other cryptocurrencies, and its fall can be fatal for many of them. If you rely on its growth in one or another aspect, what do you need to watch for?
Let's have a look over the interests of the key players: individuals, corporations, governments (regulators).
As evidenced by the growing demand, individuals are favoring cryptocurrencies. They are getting more control over their assets and more freedom (from all sorts of regulators and controllers).
Most of the companies that are not engaged in financial and investment activities have no serious reason to prefer one payment means over another. Be it mark, yena, dollar or any other convertible - it's fine. As soon as Bitcoin is technically usable, they are ready to accept it as well, which already happens.
Governments seem to be the only losers here. They are left with little or no control over a certain segment of their economy. Some authoritative governments prohibit the use of cryptocurrencies, because they feel it undermines their power and ability to control their citizens in all senses.
The conclusion is, therefore, that the governments (regulators) are likely to take counter-measures as soon as cryptocurrencies reach certain sensitivity threshold. For example, if the size of the shadow economy grows by a couple per cent because of the cryptocurrencies. While it is not excluded that some corrupted government officials also hold their assets in cryptocurrencies, but it would seem unlikely today that the most of 'corruption' money sits in such a volatile asset as e.g. Bitcoin.
Thus, one main risk for Bitcoin is probable resistance from the regulators. Justification for interference can be found easily: for example, the need to ensure transparency, to fight crime, to protect the investments of ordinary people...
The second main risk is technological weaknesses of Bitcoin. The technology is relatively immature and has many limitations of its own. Hackers don't sleep. In distinction to action by more inertial regulators, technological risks can fulfill themselves instantaneously, leaving no time for preparation.
To summarize, there is high probability of regulators' counteraction and some difficult-to-evaluate technology risks for Bitcoin and cryptocurrencies in general. Thus, if someone considers investing into them now, (s)he'd better think about extremely short term, such as days, weeks or a couple of months at most. For example, the EU plan introducing some regulations for cryptocurrencies by the end of 2017 or in the early 2018.
Image from: https://commons.wikimedia.org/wiki/File:Bitcoin_Network_Blue_1920x1080.jpg