Latency, which is simply the delay between action and reaction, has been a part of the internet since it was born. Gamers live and die by it, and not just the MMORPGs and first person shooter games. Latency is a big deal even in online chess and bridge. When your senses are tuned to quick action, especially rhythmic actions, you really notice the difference. Even when latency is small enough that you can't notice the difference, you think you can and that's all that matters psychologically.
But it's not just gamers, online commerce is affected by latency as well. Sales are lost when people have to wait, and it seems like the average attention span is getting smaller. On the blog High Scalability, there was a post recently that does a great job of explaining what causes latency and how to get rid of it. It is fairly long and somewhat technical, but worth reading. But what it boils down to is that latency comes from three things: distance traveled, and hardware processing, and software processing. The first is hard to change once the fiber is in the ground, the second depends on both how many devices the data goes through and the current state of technology for each, and the third depends on server processing speeds.
CDNs are a great way to reduce latency simply because they reduce the distance traveled by the data, but they only work for data that can be cached. Real time gamers are just out of luck, for them distance will always matter so they hunt rabidly for the networks with the least latency, whether real or imagined. The proponents of all-optical networks look to reduce the amount of hardware processing that goes on by not converting to electrical signals so often. But using such equipment is just one way of managing what is really a network design issue. On the content end, websites will use various caching techniques to reduce software processing for each page viewed. For this blog, I use a wordpress plugin called WP Super Cache, which cuts latency by saving and re-using the generated HTML pages rather than continually reprocessing the same PHP code the site is written in. It occasionally hiccups and I have to reset it, but when I turn it off people soon ask me why the site is so slow.
But for me, it all goes back to the times I spent in the late 90s and early 00s playing speed chess. When a win or loss is hanging in the balance and each move takes less than a second, too much latency is not an academic subject - it is visceral pain.