Yeah. Pretty much what everyone previous has said is the reason I, and most of us, are hating Kirkman's run. To make it simple? It's a lack of interesting things.
Interesting things. In Kirkman's world, this apparently means shock factor. Now, hey, shock has its uses, but in a world where shocking things happen all the time, shock factor tends to either fall short or set a constantly rising bar. But there's a difference between shock factor with bad writing behind it and shock factor with good writing behind it.
Examples (and yes, these are my own opinions): Brubaker killing Banshee in Deadly Genesis was, despite tasteless and mostly meaningless (despite it being pretty much the only thing done with Banshee in some time), shock factor with good writing behind it. Bru's a good writer, and while Deadly Genesis and his current Uncanny arc might have their flaws - well, all writing inevitably will.
Cable-Wolverine, Professor Xavier's secret lust, and Wolverine's bastard child, Earth as a cage... Those are all shock factors here and there, but they're all bad writing. Any of those, theoretically, could be done well. They could be made interesting. No, not all four at once can be made interesting. No, Magician is not interesting. Magician sucked. The other ideas could hold promise, but in Kirkman's unsteady hands, unfortunately do not. Might I also point out that, of those four, I know for certain that at least two of them suffered from the "shock ending" syndrome. This is the idea in writing that you start a chapter with a shock or end a chapter with a shock - and while this is a good technique when used sparingly, when used constantly it becomes a constant and boring feature. I just don't remember where in their issues Sabretooth and Lilandra let the figurative cats out of their bags.
We might all think anyone who likes Kirkman on this book is messed up, but hey, it's your opinion. I just don't know how in the world you can come to have it.

t:
As for Vaughan's run being bad... Wow. I don't know, just off the top of my head, I can think of three hugely interesting things he did - introducing Ultimate Longshot, which was done infinitely better than the introduction of Ultimate Cable (it's almost like comparing a pristine apple to a rotton one), introducing and expanding upon Emma's school and the characters there (Havok, Polaris, Cannonball, Northstar, Sunspot, Doug Ramsey) which was similarly handled much better than Ultimate Cable or Magician or Date Night, and the ending arc... What was it, Magnetic North? This arc was a huge grand finale, and set a great landmark for the title. ... Which is why I hope Kirkman doesn't touch it with a thirty foot pole.