Nobody seems to be giving a very detailed explanation, so let me explain the reasons why this episode is mostly disliked (not hated, mind you. I don't think that this special was hate-inducing, merely overwhelmingly disappointing).
Let me explain why most didn't like it.
1. Expectations
A Christmas Carol was a brilliant special. Incredibly well-written, well-directed, and well-acted. (I'd argue that it's the single best special in all of New Who, but I'll admit that's merely opinion).
In combination with this last season was, for a great many, a disappointment. The fun and magic of the fifth season seemed to be eschewed for a 'darker' more convoluted plot that ended up being pretty formulaic and not very inventive (the Doctor escaping death was incredibly obvious, and him using the Tesselecta to do so was just as blatant).
Because of these combining factors many went into this special hoping for just something magical and fun. They wanted a break from all the River and the Silence and the "Fields of Trenzalore". They wanted something fun, something whimsical, something with magic and fun.
I'll elaborate more on why the special didn't really deliver on these things, but keep in mind that for many this was the mindset going in. They were comparing it to A Christmas Carol and had just gotten out of the far-from-stellar Series Six.
2. Lack of Direction
With A Christmas Carol you get the main drive of the special right away. Amy and Rory (and 4000 other innocent people) are trapped on a crashing spaceliner and the Doctor must save them. This is literally the very first scene.
Within the first act the Doctor has his "A Christmas Carol!" moment and the audience immediately knows what this special is going to be. In order to save Amy and Rory the Doctor must "make a very mean man become nice just in time for Christmas".
We've got direction, and thus the audience gets the focus and is hooked into the story. The goal is made apparent early on and we all are on the same page.
The latest special, however, wanders a lot for it's first half.
The very first scene, of the Doctor stopping a warship and then falling to Earth, has nothing to do with the plot. All it does is provide a (relatively flimsy) motive for the Doctor to pay them back for helping him.
We don't even know what's going on for the vast majority of the film. What's with this box that leads to Narnia? What's with the tree people? What's with this kid wandering in there? We have no clue what's going on until over halfway through the actual film.
3. Bad Acting
Where Young Kazran from A Christmas Carol gives a good performance and is used exceedingly well, many of the child actors here are not well-utilized and give wholly underwhelming performances.
Where Kazran was endearing, the young girl appears irritating and the young boy appears to be absolutely clueless. The female child (who carries most of the scenes of the pair) is meant to portray an evolution from being dismissal and unhappy with her family to loving. That an exceedingly difficult arc to portray realistically and this actress simply wasn't up to the task.
4. Forced Scares
In A Christmas Carol the flying shark provided most of the scares in the special's first half. TDtWatW appears to attempt to emulate this by building up suspense and ominousness around the tree people.
Unfortunately, this comes across as very forced. The actual tree people don't appear very intimidating and most of the suspense comes from the young boy's aimless wandering (an irritating cliche that newer Doctor Who writers have worked hard to cure the show of).
This doesn't even work thematically and actually undermines the special when they attempt to make you root for the trees later on (excuse the pun). There is no "she was just hungry" moment for the trees, and as such it's difficult to connect with them.
5. The Plot Makes Little Sense
Where did the Doctor get a portal box from? When has he ever had this technology? Why would he use a portal in a gift-box and not, say, take them in the TARDIS like he's done for so many, many people?
How did the trees learn how to travel through time? Seriously, they're trees. How do they even know how to work a time machine, let alone design one? Why even time-travel at all, they just need to escape the planet. They could just rocket off the planet.
It's all a really contrived wandering through events rather than one whole story. Sure, the reuniting of the family is heartwarming and great, but it doesn't actually mean anything if none of the events are connected in a meaningful, cohesive way.
6. Most of the Plot Feels Like Filler
As I said before, the special spends half the film before actually getting to the proper plot. This is a symptom of a larger issue the special faces. It frequently prizes wandering and building suspense (which frankly, doesn't match the theme of the special at all) over actually making things happen.
This ultimately makes the special feel bloated, as if this story really wasn't meant for feature-length. Unlike the previous special where it felt like a wonderful film, this feels like a standard episode (much like Night Terrors, in fact) that's been stretched very, very thin.
7. HUMANY FRIGGING WOOMANY
It's not just this, it's all the "Moffatizations" where saying sound bite- ready lines is prized over meaningful dialogue. Much like with much of Moffat's recent work there are wonderful moments, but they don't feel like one powerful story. They just feel like they're hung together with string.
TL;DR: The special was a disappointment after last Christmas. It took too long to find it's plot and went for very long to say very little. Also, humany woomany.