I finally got back into trading recently and managed to work out one that knocked a bunch of stuff off of my wantlist:
Some of the major holes in my collection are star players from small sets. That is illustrated here. I would just get tired of adding the same three or four players all the time. So players like David Ortiz, Curt Schilling, Pedro Martinez, and Manny Ramirez have a ton of cards on my wantlist. They would appear in every single set and overshadow other players.
Add Nomar Garciaparra to that list. Sometimes I would add parallels without adding the base cards, so these three Chrome base cards at the end tended to be missing as well, but I did have multiple different colored refractors of each of them.
Most of these, I have no idea how I missed. I really like the Clement card. He started off quite well with the Red Sox before suffering a head injury. I also really like the Debut cards, always did really. Kiecker and Naehring each had some impressive moments with the Red Sox over their careers.
And finally, these are mostly inexplicable. I had no idea I was missing most of these until I set about putting my wantlist together.