Red Orc, bang-on target, as usual.
My personal contempt (if I were to use such a strong word for it) really isn't the UM's, but in "Codex Chapters" in general. As I've stated before, Chaos got stripped of it's options and general, "fluffiness," for the sake of streamlining, and streamlining was the *supposed* way of 5th ed codecis, then I took a look at the new "Codex: Fanboy" and about sold my stuff on e-bay. Since, a more even-tempered approach has been adapted, and I'm okay now.
For you folks who've played codex chapters for a while, I have respect for you, as I do for anyone who's been playing for a while, and I can appreciate where you're at when it comes to having a new codex thrown your way, it's a good feeling, there's no denying. I remember feeling the same way when the 4th ed Chaos 'dex came out (anyone else remember the abortion from 3rd ed known as, "Pamphlet: Chaos?").
The reason I jokingly (mind my humor, it's a tad dark, and a bit cynical--if you can't laugh at yourself and all that, right?) refer to the new marine codex as "Codex: Fanboy" is due to the insane amount of new folks getting into them (as with any comparison I make, according to my own demigraphic), not for the models (hell, one kid I know who got into vanilla marines hates the models), but for the power of the dex, and the power alone. One of my friends got into marines, after playing chaos for years, as he says, "I feel it's the only way I can keep up, anymore." Another guy (and an idea I've even given some consideration to) is using his chaos marines with the new marine dex as a "counts as" renegade army.
So, (for those of you who skipped my "Block O'Text" to the conclusion) I feel it isn't the UM's specifically. It's codex marines in general. Not because of any one reason, but for a few. Probably the biggest being balance and attention. I don't think the ridiculous amount of time between one codex release and the next is due to time taken to write the thing as much as it is to coincide with new model releases, which makes a certain amount of sense. But, to those who don't play codex marines, it's a kick in the teeth, seeing as we still have 3rd edition codecis floating around.
Speaking of, I have much respect for you DE and Nec players out there. Sticking with your armies, even though you've been given no love for two editions now.
Balance in all things...