The very large spreads you see here are (usually) caused by some unexpected interruption to the usual stream of arbitrage trades. These unexpected interruptions could be anything from a temporary wallet maintenance to an overloaded exchange not accepting trades for a few minutes to people fleeing an exchange due to insolvency (mt gox). If everything is working correctly, then we might get a wonderful situation where users on one exchange are just going nuts for buying a specific crypto, and then it can be very profitable but also usually short lasting. One example of this was the recent run-up of LTC's price.

When things are running "normally" the arb space is pretty crowded especially amongst crypto-crypto pairs. Things like BTC-USD arbitrage between a USA exchange like GDAX and a Korean exchange are more of a regulatory arbitrage than a trading arbitrage (it takes a lot of work to get set up to trade and withdraw money from Korea to the USA to balance the sell-leg of the arb), so I would ignore those as they are really out of the scope of a HN tech discussion.

I think this site has come up before and I might have commented on it. If you are seeing these spreads and thinking about quitting your day job, don't.

Source: I did quit my day job and paired arbs like this account for about 0% of trading volume we do.

Shameless plug: if you are interested in this stuff feel free to contact me, info should be in my profile.