Wasn't there an OF SE server not so long ago?
It wasn't particularly interesting, compared to minecraft one (and i was a bit too trigger happy in there

).
Well, as I can say, the fun of Hielor Minecraft server, apart of obvious factors came by:
-The possibility to stablish an economy ( diferent values for resources )
-The possibility to make projects with the orbinauts
-The possibility of adquiring roles on the world: farmer, miner, builder, or a lonely wolf far away from the main city
-The possibility of a certain 'evolution', as the gamers get experience they build more complex buildings and machines.
In my case I started as a simple user who had no idea about how to do a single nice house. But ended being a farmer( in the way of minecraft) who farmed iron in large quantities, had an important paper on the production of gold and ender eyes ( strange products ) and took part on the construction of the biggest structure of the server ( Sky City).
If Space Engineers has all those possibilities and/or more, I think its already enough to enjoy quite well for some months, or even a year.
This is space related and that gives other possibilites I can't imagine, I do not have this game. But I have seen some videos, and I admit quite an important number of things I have seen seems quite great.