^I think sometimes we're limited by the modern assumption that a show has to have a big unifying arc. This could've worked well as the kind of show they made in the '70s, where the basic premise was often just a loose framework for what was essentially an anthology series. Think about it. Everyone on the planet had a flashforward. That's countless different stories about how people's lives were affected by what they saw of their futures. Maybe instead of trying to run a bunch of those stories in parallel alongside this big conspiracy arc, and ending up with a rather cluttered and cursory result, they could've devoted each week to a different character story revolving around a set of guest stars, with the regular characters' search for answers being just a vehicle for bringing them into contact with those guest stars (cf. The Fugitive, The Invaders, The Incredible Hulk). Of course, the flashforwards would probably have to have been years in the future like in the original novel rather than just six months ahead, so that the heroes' investigation could be open-ended and never really lead anywhere. (But still give the appearance of advancing, like how The Invaders showed "architect David Vincent" gradually learning new things about the alien infiltrators without really varying from the crisis-of-the-week format.)
True, such an approach seems superficial when looked at as an overall narrative, and today's audiences probably wouldn't be satisfied with that. But in the '60s or '70s, the emphasis was more on the individual stories -- an anthology-like approach, as I said. And while I'm not seriously suggesting that FlashForward should've been done this way in 2010, it does seem to me that there was a lot of potential for telling individual character stories about the many ways people's lives could be affected by seeing the future, potential that was too often squandered by excessive focus on the various conspiracies and mysteries. It's a valid argument that shows in the '60s and '70s had too little arc, but I often think that modern shows err too far in the other direction.