40 years ago I'm sure email seemed like a "niche tool" used by only the technical savvy. I'm not saying that Wave will replace (or even that it should replace) email, but I do think that, especially as a younger, more technically savvy generation grows up (I'd gander that less than 10% of the students at my high school don't actively use Facebook), and as that younger generation enters the workforce, technologies like this will rise up and become a popular standard. Whether or not it will be Wave remains to be seen.

Hasn't the point always been for 'wave' to become a transparent technology? Google has their interface, but anyone can create one or just put up a different Wave server.

I don't think RSS is a niche. What major email client does not have RSS integration? What major websites do not offer RSS feeds?

I don't know about you, but the only people I know who actually use RSS are more technically advanced than 90% of the internet users I meet.

RSS is too complicated and constrained for the "average joe" on the street, which is why most "non-techies" I know just Google wherever they want to surf. Google speaks a language most people understand, while RSS still sounds like useless jibber jabber to these casual users.