HD DVD!
-It's not from the nice folks who brought us rootkits on audio CDs, millions of exploding batteries and overpriced consoles (vote with your wallet!)
-It's not yet another Sony-proprietary format that's bound to die (like BetaMax, MiniDisc, UMD, Memory Stick, ATRAC, PDD, etc)
-Blu-Ray has more evil DRM: BD+ (which isn't broken yet)
-There are FAR more set top HD DVD players than Blu-Ray ones (120000 vs 25000 last I've heard, about 5:1!)
-Blu-Ray uses Java for menus (programs!) vs iHD (simple markup, much like HTML) for HD DVD (alright, call it HDi or Advanced Navigation if you want), which is far simpler (you could make menus using notepad - if you can make web pages you can use this too), is fully documented (even stuff on MSDN) along with simulation tools and all. iHD is also cheaper. Java would require you to learn programming first (and movie studios to hire programmers), or to write some parser/interpreter in Java for some other format (slow) and such. *WAY* more complicated. Bad idea overall. And let's hope things aren't like for cell phones, where every J2ME JVM seems to have differences that makes stuff not work on 2 models, forcing them to release different versions of the same movie for different players (or spending lots of time getting your menus to work on every player). It's a VeryBadIdea™. Oh, wait, there's already
problems with BDJ? Colour me surprised

-Authoring tools for Blu-Ray were quite late (requiring drastic changes).
-HD DVD is WAY cheaper. Set top players are cheaper. In both cases, you can settle for a gaming console (if you want to be stuck with one of those) which still costs more than a set top HD DVD player. That's assuming you already have a nice and expensive HDTV. But what if you'd like to enjoy the HD goodness on your modern PC which already has a nice monitor and great sound? Just buy a drive! A Blu-Ray drive will set you back like 700$ or so, whereas you could have bought the XBOX 360 external drive for like 150$ over the holidays, and plug it using USB to your PC, and you'd already be watching HD DVDs. You're not going to find anything cheaper than that (very little investment to get HDTV goodness). Blu-Ray media is also more expensive.
-Early Blu-Ray players had image/quality problems and had pretty bad reviews (not sure they're much better now either)
-Not only Blu-Ray had bad player reviews/problems at first, and now BDJ problems, but their initial titles weren't using VC1 or H.264, and the image quality was worse than HD DVD titles released at the same time.
-HD DVD already has more than enough space as it is. In H.264 you can already comfortably fit a movie in 720p on a regular DVD (even on the single layer ones). Even in 1080p (still using VC1 or H.264) you still have way more space than one needs on HD DVD. It's already borderline overkill. More than that is just a waste (as far as movies are concerned)
-no pr0n on Blu-Ray
Why would anyone want to support a very expensive, problematic, overly complicated format that has more DRM - coming from Sony nonetheless (hence will likely be dead a a couple years anyways)? I don't know!
Blu-Ray is already dead