What would the effect be if ISPs are asked to block Bitcoin protocol traffic?
Unless you download the blockchain over and over again, finding a proxy that will tolerate your Bitcoin traffic will be fairly easy. So my guess is none.
ISPs don't currently have any incentive to block Bitcoin.
Agreed.
BitTorrent uses a lot of expensive bandwidth. ISPs block it to save money and reduce the number of expensive legal complaints that they receive, not because they're evil.
Define evil.
My ISP blocks BitTorrent traffic, and I can assure you that there wasn't a single legal complaint. (Pirated movies are openly sold on the street here, so downloading them is no biggie.)
Most ISPs block BT traffic because they like to advertise unlimited traffic at X Mbps, although they have no intention of keeping that promise. This goes hand-in-hand with other shady tactics like capping the per-connection bandwidth, which are justified by the fine print stating that X Mbps is "best effort" and you might get pretty much any speed in practice...
If your ISP is blocking Bittorrent, find a better ISP.
That might be a choice if you live in one of those fancy first-world countries. Here, we have only one DSL provider and it blocks BT traffic. You can either bend over and take it or find a way around the restriction.
Some Canadian ISP are throttling BitTorrent traffic but none block it. I guess you're american?
So Canada isn't part of America? Mind blown.