I would suggest that one of the distinguishing characteristics of cash is that it is highly liquid.  This has two senses.  Firstly, the amount of time and effort that it takes to complete a transaction, and second, the extent to which finding a "buyer" for the transaction is possible at all.

Regarding the first point: if you hold paper banknotes, this is one of the most liquid examples of value that exists.  There really is no waiting to have them transferred or accepted, assuming it's an accepted currency in the area.  You can do it in person in seconds, any time, day or night, with anyone (who accepts that currency and denomination, which would be most people in an area in which it is in use), and with no loss in value.

In this sense (traditional) bank accounts are inherently less cashlike than printed bank notes, if only slightly, because it takes some time to make a transfer, your bank card itself is not going to be accepted by just anyone (only some merchants), and it is inherently less liquid.  Where bitcoin is interesting is that in some ways it is more liquid than cash, specifically, over the Internet.  It is faster to make a transfer across the world 'instantly' via bitcoins, than by somehow trying to get the paper banknotes across the world, and much faster than completing a wire transfer (which may also be reversed).  On the other hand, when dealing with someone in person, it takes longer to receive a bitcoin transaction and confirmations than to physically be handed banknotes and make any required change, though the difference is not much.  So in terms of the speed of a transaction by someone who accepts both banknotes and bitcoins, bitcoin is slightly less cashlike.

There is a second factor to liquidity, however: not everyone accepts bitcoin for everything, so it is less liquid due to acceptance.  To get value back out of bitcoins you have just received, you need to find a specific "buyer."  To get value back out of U.S. dollars you just received, you can immediately spend them anywhere, assuming you're in an area that uses U.S. dollars, with no loss of value.  It's perfectly liquid.  In this sense, of course, Euros are less cashlike in the United States than they are in Europe, and dollars are less cashlike in Europe than they are in America.  That is somewhat of a silly distinction - to say that dollars are more so cash in America than they are in Europe.  But it does make sense to sometimes think about it this way.

And, surprisingly, we must conclude that bitcoins are more cashlike over the Internet than $100 bills are.

Chairman of the Federal Reserve of WIP