I've played my fair share of ED before and to some length after Horizons got released. True, I haven't paid much attention lately to the game's state, largely because I thought Installements and Engineers would still be way longer than announced.
My definition of "content" is quite wide. PvP is nice as long as there's team strategy, fight tactics and learning curve. Sandbox is good when the player are mostly given free movement over their world and can impact it in a meaningful way. MMO content speaks for itself because you see lots of players on them (F2P being apart, that's another consideration).
ED fills none of the above criterias for many reasons, most of which has been amply discussed in the many messages above. Still, let me explain why I consider there's not that much content brought with Horizon.
What I disliked so far from ED was that they pretend to have content while in fact many are brought by players themselves, and sometimes get aknowledged enough by the devs to become factions / regular events (exploration races or guild fights come into my mind first). Planetary landings are just another POI similar to space, quests and NPCs are a welcome change, engineers may bring some interesting interactions, though I see them mostly as another "trust point" grinding.
But most of all, the utter emptiness was what made me sad. Even starting areas don't see that much players (while I agree that having too much players and NPCs would saturate the visuals, there's certainly a middle ground). Your actions are not meaningless (speaking for powerplay) but are best when coordinated with other players outside the game, like on reddit forums. Worst of all, you can do this while playing "solo online".
I don't say I haven't enjoyed my time on ED, quite the opposite. But had I no friends to play ED with, I would have let it down earlier on.
Maybe the extension will change my mind, but for the moment I'm in a "wait and see" state until everything's revealed.