End Game Content means different things to differerent people.
In a traditional MMO, the main focus is to perform repetitive activities, in various areas, in order to reach max level in those areas.
You quest and kill things in order to gain xp, in order to reach higher character levels, until you reach max level.
You craft, gather, etc in order to reach higher levels of crafting, gathering until you can craft and gather the best stuff.
Once you "max out" your character, you are at what most consider to be end game. Since you can't gain any more character levels, the big question is:
Now what?
This is what people talk about when they ask what game content is then available for them at End Game.
Now, how this relates to STO:
STO was developed in 2 years. This is at least half of the development cycle most MMOs take, and Cryptic fully understood what this means.
It means you get a solid foundation of a MMO, but not a full-blown one that would take 2-3 more years to develop.
So, once you reach max level in STO, you basically run out of quests, and have a very few activites you can do in order to progress your character. Many people aren't happy about the lean end game. For those people, what now seems to be 1) Complain on the forums, 2) stay with it for now, 3) bail.
Now:
Cryptic is STILL developing new content, and working on it as we speak.
I personally don't feel people have any room to gripe, except that we are paying full price for what is a very lean MMO. Other than that, Cyrptic did what they agreed to do, and they did it VERY well.
Personally, I enjoy STO for what it is, and believe it has MASSIVE potential. I don't need 50 raids at end game, because frankly I don't enjoy any game activity that requires me to stay at it for 2+ hours at a time. I STILL enjoy space battles and ground missions, but I am weird like that
So...I am fine with STO, I know what it is, why it is, and am excited about what it will become.