My main problem is that I'll often buy multiple figures from a line only to get conflicted shortly thereafter and either sell or give them all away to a friend. I have a very low tolerance for things that I have that I either no longer want or (as is more often the case) I wonder why I bought in the first place. I have a few non-SW pieces I've held onto over the years, but more often than not, the non-SW stuff goes out the window after a few months. These have included:
-MAC's Buffy the Vampire Slayer line: I had pretty much every figure up through the Drusilla/Gentlemen wave, but once Seasons 6 and 7 came out on UPN, I became kind of disgusted with what I thought was the flogging of the show for no apparent reason. I went ahead and sold the entire line to our very own Ben (Inadvertent Imitation) as I really had no interest in the toys once the show, in my own opinion, had jumped the shark.
-Toy Biz's Spider-Man Classics line: I just remember being fascinated in 2000 that there was a line of highly detailed action figures with gobs of articulation, and since I was a Spider-Man fan growing up, I figured "what the hell?" I still think they were great figures, and I only bought a few (red/blue Spidey, black suit Spidey, Daredevil, and someone else I can't recall), but they were caught in one of my grand purges and went the way of the dodo.
-Playmates' World of Springfield line: Although it's more a case of only having the characters I really want(ed), there were several characters I would have loved to have had figures of but never got, mainly due to the whole overpriced "playset" factor. I probably would have gotten every figure in this line had characters like Marge, Maggie, Sideshow Bob, Jasper, etc. been part of the basic figure line and not made as pack-ins. As it is, mine are now boxed up (with the exception of my carded Troy McClure and my loose Hans Moleman) and in storage.
-Transformers Classics: A more recent entry, but one that I am pretty sure I'll be hanging on to, if only for the nostalgic appeal. I didn't get the little Mini-Con things, but I did get all of the basic stuff based on the old characters, along with the TRU Soundwave and the WM Devastator set. Still, because of room they're in Rubbermaid shoe boxes, but are one of those things I'd display in a second if I had the space to do so. That said, they probably don't belong on this list.
-Hasbro's SW 12 inch line: The only SW stuff that I truly regret getting wrapped up in at all. I have a lot of this stuff, and I'd get rid of 99% of it (read: everything but the bounty hunters and Jango Fett) if I could find a buyer. Unfortunately, since they're all in storage and would be nearly impossible to sell on a piece-by-piece basis, they'll probably remain in storage for the foreseeable future.
As for non-SW stuff I've gotten that has stayed with me for many years now and will most definitely not be sold, those pieces are limited to the Marvel Legends Series III Magneto, a McFarlane Toys Leatherface figure from "The Texas Chainsaw Massacre", and a McFarlane Toys "Danger Girl" Abbey Chase figure (I'd love to have the other two figures as well at some point).