My mom is an ISTJ and she has a LOT of stuff.
She's much more into knick-knacky things than collecting letters or highly personalized things. She has a lot of figurines (china, hummells, porcelain), a lot of crystal, a collection of perfume bottles (her newest collection), collectors plates (I can't recall the brand, but it's something where a new one comes out every 5 years and she has at least 20 by now), all kinds of other little things she picks up here and there...rather expensive taste, although she also gets stuff from craft-fair type places on occasion.
She doesn't save newspapers or anything like that - she doesn't save just to save - she just seems to like having a house with a lot of stuff on display. She'll occasionally give older decorations that she doesn't like anymore to goodwill; so I know she doesn't have a problem with hoarding.
Jennifer, your mentioning ISFJ's and scrapbooking makes me laugh -- my ISFJ friend LOVES scrapbooking. That's actually her main hobby!!
As for myself - I suppose I have one 'collection' that I can acknowledge. I have a small collection of wooden bird carvings. So if I ever come across a bird carving in my travels, I pick one up. So I tend to like more hand-crafted things, and more 'natural' art. Woodworking, fabrics, stuff like that. Oh, one more collection -- although in my defense, my mom started it for me.

I have 6 little jewel-boxes that are all animal or bird-shaped. My mom has given me one every year for 6 yrs at x-mas. They're awfully cute.

So I guess I'm not immune to collecting myself! I just have no desire to accumulate as much 'stuff' as my mom, and don't want any more collections!!!