Paperwork. We had BOXES of old bank statements, returned checks, etc. We even had my mom's stuff, and she died in 2005! I finally convinced DH that it was OK to get rid of this stuff (in a locked, shredding bin at work) and also that we don't need to keep hard-copy credit card statements anymore. As long as I make sure the account number isn't showing, he has finally agreed to get rid of all this kind of thing (well... except tax returns, he wants 10 years' worth kept).
Books. It breaks my heart to say this, as i have been a collector of hard cover books written by my favorite authors for decades. I have/had some entire series collections, in hard cover. Unfortunately, I can't read books anymore. My eyesight has changed to the point where there is no good, comfortable way to hold a book for me to be able to read it for more than a few minutes, and you can forget about reading in bed. I have swapped over almost exclusively to reading on my Nook. I LOVE being able to read regularly again, but I mourn not reading from a nice, hardcover book. However, they take up space and I know I won't read them again, so I'm slowly parting with these old friends. It's hard.
"Unused" things. I am a big keeper of things that I might need, some day. Also, I don't see the point in discarding something that might, one day, be useful again. Consequently, we have a lot of stuff. I am tired of all this stuff and am trying to declutter, but it's hard. Maybe it's my Yankee roots, but it's hard to get rid of things that I don't use, but still have life in them, even if I am donating them. HOWEVER... as I said, we have lots of "stuff" and as I get older, it seems to have less value. Also, things are cheaper than they used to be. I remember my mother ironing wrapping paper and saving gift boxes. They were expensive to buy, so you kept and reused this kind of thing for as long as they were whole. Not anymore! I can get shirt gift boxes in multipacks for $2; no way am I carefully opening, refolding, and saving this kind of thing anymore! Hopefully we'll get through all the decluttering in the next few years... DD is already threatening to come in with a dumpster, as, like she says, there is no way we are leaving her all our "stuff" to sort out, as her inheritance!