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The historical significance and key features of cryptocurrencies are vast and diverse. This digital asset, designed to work as a medium of exchange, has revolutionized the financial world. Its primary functions lie in using cryptography to secure and verify transactions and control the creation of new units. Cryptocurrencies leverage decentralized control as opposed to centralized digital currency and central banking systems.

Historical Significance of Cryptocurrencies: Cryptocurrencies emerged as a side product of another invention. The mysterious figure, Satoshi Nakamoto, who is still unknown to the present date, initially invented Bitcoin, the first and most important cryptocurrency, not to devise a cryptocurrency but to create a "decentralized digital cash system”.

One of the most notable aspects of cryptocurrencies’ history is the reason they were created in the first place. During the great financial crisis of 2008, people’s trust in financial institutions markedly dropped. Nakamoto intended to create a system where trust in third-party institutions wasn’t necessary for transactions, thereby creating a system where “trust is replaced by cryptographic proof.”

Key Features of Cryptocurrencies:

1. Decentralization: They are not controlled by any centralized authority, such as a government or financial institution.

2. Anonymity: Cryptocurrency transactions can be conducted with the anonymity that is similar to that of cash transactions.

3. Availability: They are readily available to the general public. Anyone can use cryptocurrencies for transactions provided they have access to the internet.

4. Irreversibility of transactions: Once a transaction has been validated and added to the blockchain, it’s almost impossible to alter or delete that transaction.

5. Limited supply: Most cryptocurrencies have a limited supply; for instance, the total amount of Bitcoin that will ever exist is limited to 21 million coins.

Understanding the historical significance of cryptocurrencies gives us a broader understanding of the current financial landscape, while the key features state the advantages which stand to benefit industries and individuals alike. Blockchain technology has not only broadened the scope of digital currency but also offered a wide and diversified playground for investors and traders. However, like any budding technology, it comes with its own set of challenges. As we brave this new financial world, it's important not to get swept up in the hype and to be aware of the risks and rewards involved in investing in cryptocurrencies.