First off, and probably most importantly, Mirai Nikki is a psychological horror manga. The anime, on the other hand, is very action-oriented, with such an emphasis on the action that it takes away completely from the atmosphere. This is one mistake the anime made. The manga allows you to get a much better understanding of the adrenaline rush you get from the main characters, and what makes them so interesting is your ability to relate more to their problems - their human aspects are ultimately lost in the show. This immediately takes the story down a few notches, a hindrance it absolutely cannot afford, especially when it's already sort of a jumbled mess, and especially when the pacing is totally off.
The art style is very generic. At first I thought it was incredibly well done, but after reading the manga, I can't help but feel that I've seen it somewhere before. It almost seems like a carbon copy, and that's never good. Anime was designed with breathing new life in mind, and if it can't do that, it fails at being a faithful adaptation. The sound work is even worse; aside from the opening theme, it almost seems like the entire time, it's trying to confuse you. Some of the soundtrack and character voice work do build up suspense, but it turns out to be somewhat of a cheap scare as a result. They're not too keen on the uptake, either; you can instantly tell exactly what character is supposed to be doing what by their voice acting, and it all seems kinda stereotypical and bland. That said, what this anime does excel at is what its manga excelled at as well; it's a very charismatic and clever combination of things. I didn't really fall in love with the individual aspects, but I absolutely loved how it all came together. This allowed me to enjoy it probably a bit more than I thought.
Overall, it's not a terrible anime; if you're an absolute critic, you can tear it to shreds and absolutely hate it, but if you're looking for entertainment, this may or may not deliver what you're looking for. I'm still going to keep watching it myself.
