What I mean by more item sets is things like the starter set, even though the whole thing is given to you free, sets would be suits of armor, or pieces of armor and weapons that all have the same theme (like the starter set). Each piece of a set should have to be gotten individually, through promo or mob drop or even special events. This isn't necessary and I'm not sure if its possible, as I have little experience with servers, but have the entire set give a special bonus would add a good motivation to collecting the entire set. I just thing this would make collecting rare items more fun, and more satisfying if more items were related.
We have Starter Items, 60k Member Items, and Marlix Set. Hell, we have IcecreamCow's birthday event items! More will be added when Aikar feels like it.
I was thinking of less one-time promo things and more stuff that you had to work for to get, but I am aware of all the current sets no need to tell me what they are.