[Ethereum] Why is Geth’s “fast” sync now the default, whereas before it wasn’t

There's a lot of detail on this PR on github. Here's a quote:

Instead of processing the entire block-chain one link at a time, and replay all transactions that ever happened in history, fast syncing downloads the transaction receipts along the blocks, and pulls an entire recent state database.

Two reasons:

• Fast sync indeed has one additional weakness compared to full sync. As long as an attacker can keep you isolated from the main chain (e.g. infect your router), it may construct an arbitrary state that your node will believe (details in https://github.com/ethereum/go-ethereum/pull/1889, Weakness section). This is the reason why fast sync is only a one shot sync mechanism: people will likely verify that they are indeed up to date and correctly when they initially sync, but probably will assume nothing's wrong afterwards. It's hard and expensive to trick fast sync, but given "enough" trials, you could eventually do it. By disabling fast sync after the initial one we're making it impossible to mount a realistic attack against it.
• Fast sync does not retain past transition data (e.g. what an account held half a year ago), only a relatively fresh view of the network (last 8-10 hours) when it finishes, (from which point it will act as a full node). Because of this, if we would enable by default then it would be bad for block explorers.

However, hopefully Geth 1.5 will have state pruning implemented, with which we're going to make the "archive" and "full" node separation. Then archive nodes will have pruning and fast sync off, whereas full nodes will have pruning and fast sync on. At least this is the plan :)