I have a Late-2010 MacBook Air that has an Core 2 Duo, with 2GB RAM, and Yosemite runs absolutely fine. I currently have 8 tabs open in Safari (one tab is Netflix), Mail, and iTunes open and my system is not sluggish at all. I think in systems like mine, the SSD has a lot to do with why things run so smooth and also as someone said in a comment above OS X is EXTREMELY good with RAM management.
In fact I just replied to a post above about a fellow member wondering if his MBA with a Core i5, and 4GB RAM would run Yosemite fine. Clearly I told him yes as it absolutely should.
As I told him, I occasionally have to run the Purge command in Terminal but that is very rare. I very, very, very rarely get system hangups...in fact I could not tell you the last time I had one. MacBooks are built phenomenal and OS X is just a superior operating system on most all levels, but especially RAM management.
So I do not know why you are laughing, because my system runs absolutely fine and I am sure there are plenty of other users in these very forums who use Yosemite on the same system I have or other systems with only 2gb RAM that might even be older than my Late-2010.