Let's start with the big picture. The best breakdown of the key points of this merger I found was over at GameSetWatch. The biggest key, I think, is the huge impact of World of Warcraft on the financials of parent company Vivendi: There's a reason why Blizzard have been and are left well alone - the clout that comes with this mindblowing statistic: "Blizzard Entertainment [which has "over 9.3 million subscribers" to World Of Warcraft] has projected calendar 2007 revenues of $1.1 billion, operating margins of over 40% and approximately $520 million of operating profit."
Another point stressed there, and reiterated in Blizzard's FAQ on the merger, is that Blizzard is going to maintain its independence. You won't have to worry about any crazy changes to World of Warcraft as a result of this deal. Blizzard's president Mike Morhaime sat down with 1up to talk things through, and he even went so far as to say that the company is still fairly cold on console gaming. Don't expect a slip in quality from solely Blizzard-branded products.