To be clear, I understand that "new from the publisher" tends to lose its meaning in regards to comics -- even if you can't find a new copy online, there's likely dozens of local comic book stores out there with a never-read copy of whatever trade paperback, old or new, you might like.
I also understand that just because we might argue within the confines of the Collected Editions blog that The Final Night, for instance, ought still be in print because it reflects a chapter in Hal Jordan's life as Parallax, does not mean there's a real widespread interest in Final Night, nor that it would be profitable long-term for DC Comics to invest in a second printing (edit: nor, as Johanna and Caleb point out, would it be profitable necessarily for booksellers to stock it).
That said ... It's astounding to me that some of the titles below are out of print, and these are in fact just culled from a general survey; the fact is probably that many, many more titles are out of print.
- Batman: Legacy, out of print, along with Batman: Cataclysm (the volume that preceded these, Batman: Contagion, is still in print)
- Batman: No Man's Land Vol. 3 and Vol. 4, out of print (but volumes one, two, and five are available)
- Flash: Rogues, out of print, from Geoff Johns' Wally West run, along with Crossfire, Blitz, and Ignition, (being the third, fourth, fifth, and sixth volumes) but Wonderland, Blood Will Run, The Secret of Barry Allen, and Rogue War are all available (volumes one, two, seven, and eight respectively)
- Nightwing: A Knight in Bludhaven, out of print (the first volume, as is Rough Justice, Love and Bullets, A Darker Shade of Justice, The Hunt for Oracle, and Big Guns, which are most of Chuck Dixon's run on the book)
- The Death and Return of Superman hardcover omnibus, out of print
- Underworld Unleashed crossover, out of print, along with Final Night
- Black Adam: The Dark Age, out of print
- Countdown to Final Crisis Vol. 4, out of print, the lead-in to DC's second most recent crossover event, while volumes one, two, and three are still in print
- Flash, the Fastest Man Alive: Full Throttle, out of print, being the better of the two books, whereas Lightning in a Bottle is still available
(Amazon reviewer Kauffinbauchser has put together an additional list of out-of-print DC Comics trades. Some of these are right, some have been subsequently republished.)
Now, a book like Batman: Legacy (in the vein of classic Ra's al Ghul stories, but maybe not a classic itself), I can understand being out of print. Even the Death and Return of Superman Omnibus, published for the Superman animated movie while the individual collections within are still in paperback, I can understand. But a book like Countdown to Final Crisis Vol. 4, published in 2008 and a factor (if ultimately minor) in Grant Morrison's Final Crisis? Very surprising.
Obviously, DC Comics history is not history itself, and DC's under no obligation to make its universe's whole history readily available to every reader. In addition, I recall it took me a number of years of searching before I found a New Teen Titans: Judas Contract collection to read it for the first time, and some might argue that the search is part of the fun, or part of comics fandom (that selfsame story was subsequently reissued, is no longer out of print, and is indeed now readily available).
But for those interested in the "full story," and given the rise of interest in trade paperbacks and the number of readers I hear from who've "gone back to the beginning" to follow the DC Universe in trade form, it's disappointing that seminal crossovers like Underworld Unleashed and Final Night shouldn't be easy to find. That one can't read the ground-breaking Batman story No Man's Land, which includes the beginnings of the conflict between Two-Face and now-Question Montoya, is equally sad (if ever a series deserved omnibus collecting, it's No Man's Land). And that as a new Flash series begins, the collections of the well-regarded previous Flash series by the exact same writer aren't available? Surely there's some profit to be made in those books being around.
I'm not much inclined to start reading my trade paperbacks on a digital device, but I can see this as one area where digital comics hold a lot of value. If DC doesn't see the value in a new printing of Underworld Unleashed, I understood, but if at least there was a venue for one to legally purchase that story online and read it, if even just for the historical benefit, I think that would really be something. Imagine, every comic book DC has ever published, cataloged and available for immediate download. Just imagine.
From my perspective, I had thought at one point about skipping the Black Adam: The Dark Age paperback when it came out and picking it up later on, and I'm glad I didn't -- it quickly went out of print, and now sells for double or triple the price online. My growing awareness of the unpredictability of DC's trades going out of print has made me much more circumspect at my local comics shop -- this is something I'll continue to keep an eye on.