Yeah, flannel-lined, polyester-filled sleeping bags are the most common, large item left behind. Depending on the setting, I suspect teen-aged lovers or someone who wasn't going to camp again and didn't want to carry it back.
Glass bottles. When left in a USFS cabin, I imagine someone rationalized it as, "Someone could use it as a drinking cup or candle holder." But really, it was heavy trash they left behind.
Hunting camps will get things brought in but not out over the years. Some woodcraft stuff - tables and chairs from saplings, some thing flown or hiked in. There does seem be enough "pride of ownership" that they don't leave trash. And in bear country, they don't leave food waste.
A lot of things I'll carry out. Especially if it's my last day out.
Edited to add: walking my beach there is lots of sport and commercial fishing gear. My best finds were a Casio dual-time-zone, analog/digital wristwatch. And a Leatherman in good shape. On another beach, we found an upper-end halibut rod. In Prince William Sound we found a fiberglass kayaking paddle. We returned it to the kayak rental company who said the last renter would be happy to have the $200 charge reversed.