Quote from Wiki:
In economics, diminishing returns is the decrease in the marginal (per-unit) output of a production process as the amount of a single factor of production is increased, while the amounts of all other factors of production stay constant.
When people talk about diminishing returns in swtor, what they mean is at a certain point it becomes less advantageous to continue stacking a given stat because it ends taking up more and more base stat (crit, surge, etc) to increase the actual output percentage (the 30% you see in your character screen).
This has to do with in game mechanics in relation to how stats are calculated.
It has nothing to do with gear; just because there's a new gear set doesn't mean that the place where you start experiencing diminishing returns for a stat has changed in any way.
Edit: Also, this is totally the wrong forum.
You were better off posting in 'General Discussion' than 'Star Wars Discussion'.