To be honest, I haven't played much since the release of .87, for what the lag was the main reason.
I had a perfect ping of 25 and never ever experienced any lag within over 3 years of constant playing. Huge Outcast-Corsair skirmishes, massive Rheinland-Liberty KRIEG and huge Order-BHG battles were possible without having one of the up to 40 ships jumping just once. Ok, that was all from .85, but in .86 the lag was still bearable and I hardly noticed it as long as I didn't joined a base siege.
That much for the lag, a lot of balance changes happened that somehow influenced my fun at playing. Can't deny that.
Plus, there have been system changes/removals/additions which obviously changed the Sirius I got to know and love. I call it "nostalgic bitterness".
Then there's of course a lot of friends/players that left Disco for good. People nobody else could ever replace. They left a black hole in one's Disco heart and it'll stay empty no matter what.
Another point (that at least applies to me) is that some of your RP characters feel to be "finished", and it's exhausting to keep their roleplay going, while I can't just keep on playing with them if there's no real progress in their stories. Just bland.
Also, there's the "virtual wealth/achievements" which are limited. No more moneysinks that you feel your time being worth it. In fact, at one point you'll know all the systems, bases and planets by their names, their infocards, all the factions' lore... there seems to be nothing new for you. Can't continue reading a finished book.
That leaves the only option to write roleplay yourself. But as said - after some time even that gets bland.
Now I can't remember my "red thread" ... To draw a conclusion; I suffer a "Disco depression". Something still pulls me back now and then, and I'm still interested in the game and community, but I'm somewhat bored and unimaginative. And seriously, that lag delivers the final nail blow.
In addition, life is being awesome at times and there's always more to do. With rewards.