Admittedly, what I have to say is just more unique, idiosyncratic individual experience, but I'll throw me two cents in!
I came to D&D just as 3e was ramping up. At the time, the Forgotten Realms Campaign Setting guide was coming out, and the Greyhawk Gazetteer was
nowhere to be found. Seriously. My parents owned a bookstore at the time, and Wizards (and their publisher who, at the time, I believe was Random House) were not doing more print runs of the Gazetteer. From what I learned in that business about how WotC's publishers handled stock, that meant that Greyhawk probably didn't sell as well as the FRCS.
For me, personally, (again, growing up in a bookstore) I ate up Forgotten Realms. I met Ed Greenwood and Bob Salvatore at booksellers conventions, and I gobbled up their novels. I could easily set campaigns in those worlds because the campaign setting was vast, detailed, and very user-friendly. The encounters I had with Greyhawk lore through the Core 3 were distant, muddled, and ultimately bland because there were no stories associated with them for me.
Having said that, I did read those 3e Greyhawk novels; I still own all of them and loan them out to my students who are looking for a good, brief, fantasy romp. My only criticism with those books as "set-pieces" is that they (once again) were entirely generic. The settings/places were quickly forgotten, and the only thing binding them together were the common character threads.
From a business perspective, I think WotC made the right choice with Forgotten Realms being the initial flavor of 5e. Is it still my favorite setting? Do I still eat up books by Greenwood and Salvatore? No.
From a rules perspective, I think WotC made the right choice with the
multiverse. Having said that, I do absolutely see Forgotten Realm's influence as a "default" setting. It's not the same intensity of default as Greyhawk was for 3e, but I think an argument can be made for it. Most of the examples in the PHB/DMG/MM reference FR; they also often reference other settings, but FR seems to always be mentioned.
Now for the inevitable (again, idiosyncratic) pros & cons list!
Pros:
- FR has a wide media appeal: games and books galore!
- Personally, all of the players in my gaming group have played in FR before. We have been able to learn the new rules without worrying about a new setting.
Cons:
- GH fans feel left out in the cold; where's the love?
- 3e established a precedent of GH being the lens through which the game is viewed; WotC has gone even further away from that model.
Mixed:
- The staggered release leaves everyone without a polished Campaign Setting (or knowledge of if one will ever be released for the new edition).
- Personally, I'm a huge 3e Eberron fan and am hoping for some Eberron love; however, I do NOT think Eberron should/could be the default setting for D&D, so I'm understanding that I need to have patience.