[quote name='cdietschrun']
But the flash sideways are CLEARLY an important part of the ending of the show, they are not just time fillers. .[/QUOTE]
That may be, but that doesn't mean that watching half an episode of Jack's imaginary son or Locke bumbling his way through cameo appearances wasn't boring as

.
[quote name='hufferstl']xycury: Can't the same be said about the Flashback stuff from the first 4 seasons? How much does Jack's tatoo girlfriend really pertain to the events of the island, or Kate blowing up her dad? To me, the show is about the characters and how their decisions on the island either reflect or contrast their previous life. [/QUOTE]
The difference is the flashbacks showed us backstory on these characters, or put their actions in context. The tattoo episode was weak, but Kate blowing up her father was something that totally changed her life and made her into the person she was on the island. If not for the flashbacks, we wouldn't have understood why Locke was so quick to believe in the island, why Sawyer was such an arrogant, standoffish guy, why Kate was a bitchy criminal, why Jack felt the need to fix everything, why Charlie and Boone were so desperate for acceptance, why Ben became a total monster, etc. Sawyer killed Anthony Cooper wouldn't have mattered nearly as much if we had just been given throwaway lines about Sawyer's dad killing his mom/himself, or about Locke's dad being a dick. Des and Penny reuniting in the S4 finale wouldn't been as amazing if we had only heard of Penny and not seen what their relationship went through. Most flashbacks were needed to tell the story, even if there were parts when they retreaded the same ground.
On the other hand, what do the flash sideways show us? That if Jack had a preteen son he would be a bad parent? That if Jin/Sun had been just sleeping together instead of getting married they would have been happier, but Sun would end up shot? None of that means anything to me. If the sideways were just "this is what would happen if the plane had landed", that would be fine. But it's not. It's "this is what would happen if the plane had landed, and if the characters' lives were totally different at random."
Will the sideways be necessary to the plot eventually? As a whole, probably. But arbitrary differences in the sideways lives and character cameos aren't going to be important, so all that feels like a waste right now. We've had half an episode devoted to the importance of the AU and nine episodes of what-if scenarios. That's not a good ratio for me.
Plus, it sucks that so much time is devoted to this that the island plotline, IE the story and characters I've been following for six years, has been stuck at "everyone sits on the beach or in the jungle and waits for Flocke/Jacob/Richard to tell them what to do" for half the season.