Here's the scenario:
1. Buy BTC and ETH on CB with USD, hold them.
2. Months later, buy BTC/ETH, transfer to another exchange (Bittrex/Binance) and immediately buy an altcoin and hold.
So (1) isn't a taxable event because I never sold them (unless I'm forced to use FIFO reporting, then it gets murky - but my lots are very clear/specific).
(2) apparently is taxable, but since I did the entire transaction within an hour, do I still need to report it? The gain/loss would be tiny since the entire lot transaction was done within an hour (usually less).
Prior to the tax overhaul that was just passed, there was an open question about like-kind exchanges -- where you roll your investment into a similar investment without incurring a taxable event. The overhaul ruled out like-kind exchanges for cryptocurrencies (and most assets), so we're supposed to treat (2) as a taxable event.
The gains/losses are negligible in the example above, so it shouldn't really matter.
The point of like-kind exchanges is to avoid resetting the clock on reaching long-term capital gains. So in the above example, people use the lots bought in (1) to buy altcoins, but they keep using the original BTC buy date for determining long-term vs. short-term tax rates.