First off, let me say, I'm not ripping on the second because it "didn't live up". The review is simply based on the overall, actual mechanics and execution which I felt was extremely poor; especially compared to the first season. I was a huge fan of the first season, mainly because the production was of such high quality. Regarding the second season, that quality simply isn't there. I will explain in detail in each subsection while keeping it spoiler free.
Story:
The first half of season 2 has no consistent sense of time; the narrative jumps all over the place leaving the viewer confused and agitated. Essentially, I felt like the beginning half was just a series of subplots stitched together haphazardly. I have never felt more agitated and irritated watching an anime. The root cause of this agitation was simply because the narrative refuses to give you any relief of anticipation. It just keeps building tension and more tension without giving the viewer any release or even any expectation of a release. After about the halfway point, the plot finally starts unfolding but honestly, not even in an interesting way. Literally, the least interesting way possible (you'll know what I mean). As a result, it felt like the action sequences had no sense of purpose anymore. Also, any sense of character unity that was prevalent in the first season starts crumbling apart. In short, the second season has some serious pacing problems coupled with uninteresting plot development. It's like the writers were trying to make the plot interesting by artificially injecting a sense of anticipation and the result is just a viewer who gets let down.
Art:
Given the spectacular art in the first season, I did NOT think the second season would pull some of the s*!t it did. Excessive use 3D models with terrible motion on top of it. Dark, muddled environments gave it a medieval look... but honestly, RuneScape did it better.
Parts of the anime are gorgeous while other parts look worse than a video game from the late 1990s. The colossal titan was modeled and animated fully in 3D at one point and it looked terrible; my jaw dropped. This is not even close to the same quality of art as in the first season. The 2D art maintained its look but I could see that they did their best to cut corners. Less landscapes, less close ups, less color contrast and just less complex 2D animation overall. Towards the end of the season, that began to change but for 3/4ths of the season it's pretty obvious.
Sound:
This is the one area that kept up well and probably actually outperformed the first season in my honest opinion. The beautiful piano etudes during emotional (and action) scenes, the clever use of shepard tones to build tension and anticipation during down time, the music kicking in right at the perfect moment of action; the type that gives you goose bumps. Attack on Titan 2's music and sound team pulled off a great feat and honestly, it's one of the only reasons it kept this anime in the "epic" category for me.
Character:
As I mentioned in the story part above, the one of the biggest downfalls in the plot is the lack of character unity and character purpose. Only a few supporting characters really get developed while the rest are forgotten about and get barely any screen time. The whole aspect of a character using their "cleverness" or "what makes them unique" is thrown away for a titan brawling s%^t-show. It's the same trap that many shonen series find themselves in as their plots progress. It goes from being "weak character(s) using clever, interesting solutions" to "ridiculous OP characters using raw power and all the useless, supporting characters watching in awe".
Enjoyment:
I enjoyed watching it. I wasn't particularly upset with the overall plot. I just wasn't expecting the series to turn out the way it did and it didn't have the same animation and story telling quality I was expecting from the first season.
Overall:
Fun to watch and glad to get more of the story. It has it's disappointments being plagued with pacing problems, bad art direction, and lackluster character development. It it makes up for it with the epic nature of the plot and with some seriously fantastic sound and music design/execution. As such, i'll give it an overall rating of 4 (Decent).