Part of me wants to ask: Wouldn’t the best partner for either Microsoft or Yahoo! be a traditional media company? But then, that never worked for AOL like it should have. Some observations:

– Audience. I haven’t done the analysis yet, but both MSN and Yahoo have such large, general reach in the US, I doubt there will be much addition. Both are general-purpose portals with strong e-mail bases — we’re not talking about putting a lot of specialty niches together. Yes, there will be much more total time-spent or page views, but that’s not the problem. If either company could monetize their remnant inventory really well, we might not be seeing this.

– Search is another story. Presumably, Microsoft would get instant reach for its search technology, and will satisfy those who have been advising Yahoo to get over it, and just use somebody else’s search — though “they” usually mean Google’s. They’re talking about scale economics on the concall, but both portals already have scale in display. This is totally about search. Neither company has scale outside their own properties for their ad networks or exchanges.

– Microsoft has never done a merger this big before. Last time Yahoo did — Overture — it took a loooooong time. And there may be white knights around, I suppose, since it’s unsolicited.

– Of the other biggest trends in US online media (see Figures 4 and 9), let’s just say there is room for improvement. Neither is a leader in online video, but neither is anybody else except YouTube. Neither have done as well in social media and marketing as it should have. Both have the capability, through strong sales forces that have access to resources to create original marketing/content integrations — call it branded entertainment, sponsorships, custom ads, whatever. But at least some of the time, they’ll need to apply these skills outside their own properties.

– On the horizon, mobile might just be the game-changer.