I'm a fairly serious collector in some senses, and not so much in others. I own either a printed copy or a (legal!) PDF of almost every D&D game product ever sold. I originally started collecting D&D products because I was attempting to create a complete index of all D&D monsters, and that made it necessary to have every product that might contain a monster. But I wasn't fussy if that was in print or PDF. Eventually I finished the monster index, and since then, I've been gradually filling in the gaps in my print collection.
For 3.X and 4th edition, I have copies of everything available for sale except the Deluxe 4e books and the Power Cards but I don't have a complete set of adventures distributed via the RPGA. For 2nd edition (which includes a truly scary number of products), my collection is missing only the Player's Packs, five Dragonlance supplements and some of the Decks of Magical Items/Encounters/Psionic Powers. For 1st edition, I'm missing five more Dragonlance supplements and three of the H-series adventures. For BECMI and earlier D&D, I have only about 25% in print, so plenty to still accumulate there.
I have complete print sets of Dungeon, Polyhedron and IMAGINE, but my Dragon collection only goes back to about #60 (but I have the CD-ROM collection of Dragon PDFs). I don't own any miniatures predating the pre-painted series, but I have more than 5000 of the pre-painted figures. I own very few of the Forgotten Realms and Dragonlance novels, but for most of the other campaign worlds, I have the novels too.
Eventually, I think I'd like to have a "complete" D&D collection for a reasonable value of "complete". However, over the years, there have been so very many things produced bearing a D&D logo, that I'm not sure if there is anyone on the planet who could claim to have a truly complete collection. (Or even a complete list of everything ever made.) For the moment, I'm happy to settle for having the largest D&D collection on one continent, which, as far as I know, is true.
Like DaveMage, I'm not interested in different printings of the same product. As long as the content is the same, I'm good with just one printing. Unlike DaveMage, I do aspire to eventually own a copy of ST1: Up the Garden Path. (But I'm not interested in the pinball machines...)