The only thing big about this deal is the money Microsoft will be paying. Yahoo has been on a steady decline. I recall reading a statistic that notes Yahoo's marketshare recently dipped below 20% for the first time in it's history. Yahoo of course is the distant second place behind Google.
John Sicracusa over at Ars has written a nice piece on this issue.
http://arstechnica.c...eep-in-the-dead
The bottom line is that Microsoft doesn't have much to offer Yahoo. What they need to fight Google is more than just money. They need vision and new ideas. Microsoft is not the one to help in that matter. As for Yahoo, Microsoft will swallow them up, get rid of the rudundant stuff, convert what it keeps into Microsoft based technology (like it did with Hotmail), etc. The best and brightest from Yahoo aren't expected to hang around.
Microsoft in turn will get a quick boost in marketshare, but at what cost? Will they be able to maintain it? Doubtful. Two losers in a market don't add up to a winner when you merge them. We all know Microsoft has money to burn, but I'm confident history will look back at this as being one of the more costly blunders Microsoft has made.