769 is just a blob fest, and I find that boring. You've got a couple giant Catholic blobs, a couple giant Sunni blobs, and an Orthodox blob. They either stick around all game being boring blobs, or the Catholics break into ugly messes of gavelkind nonsense. Not to mention the raiders EVERYWHERE, being obnoxious without really adding much fun. There are no real marriage partners either, which means you have to wait a generation before you can do the crazy inheritance shenanigans that are one of the best things about CK2. Besides, no one actually plays to the end of the game, so "more time" is basically an illusion. We'll leave aside the historicity issues, because 769 is best treated as a fantasy scenario, and I do play AGOT or Elder Kings on occasion, so I can understand people who like fantasy, even if it isn't what I'm looking for in vanilla.
867 vs. 1066 (or later; I actually like post-Manzikert starts for the fun of seeing the Byzantine Empire actually in trouble on occasion) comes down to how I'm feeling in terms of balancing the annoyance of never ending raiders vs. the annoyance of dealing with the HRE. There's also the issue of whether I feel like playing as any particular character or not, which determines a lot of things (can't play as good old fratricidal Alfonso VI if you start in 867). And of course, playing as a merchant republic in the early start dates is an exercise in futility.
More broadly, the earlier start dates (especially 769, but also 867 to some extent) tend to be dominated by blob-on-blob warfare and inter-religious wars, which I find less interesting than the intra-religious conflicts that dominate the later start dates. Wars in CK2 are actually fairly boring compared to the scheming that leads up to those wars: getting CBs and interacting with your vassals/co-religionist neighbors. It's also one reason I'm not a big fan of reformed pagan games; even leaving aside the ahistoricity of it all, you quickly end up playing as the only blob of your religion, holy warring your neighbors in a "raise levies==>march to target==>conquer==>disband levies==>prepare next war" cycle or (if you play with defensive pacts) insert a decade of waiting in between.