Story: I do enjoy slice of life but it can become boring if not done well. Hanasaku was definitely done well. It was very realistic but had enough drama, romance, comedy and wit to keep you wanting more. The plot doesn't go at a snail pace but is well developed without feeling like too much at once. There is some unpredictability which keeps you guessing. There are a few filler-ish episodes which brings down the series a bit but only a by a bit. It's definitely suitable for a wide range of audience without being too much in one extreme. Overall, it's a refreshing series that has plently to offer.
Animation: Beautiful. Impeccable. So much attention to detail but not to much that you become overwhelmed. Everything from the background to the character designs to emotions was beautiful. Just go watch it. Your eyes will thank you.
Sound: The soundtrack was nice but I can't really remember too much of it. I liked the openings and ending. The seiyuus were fantastic. They embodied their characters to a tee and even added to the scene which I have trouble finding at times.
Characters: Surprisingly well developed and diverse without being bland. The characters have a wide range of personalities and it's nice seeing them work together. I thought I wouldn't like some of them but they grew on me. They are a fun cast that I would love to meet in real life. Although they remain fairly realistic, they don't bore you.
Overall: Definitely a series I would keep in my collection. It's light and refreshing but has depth and wit. Also, a fair amount of humour which will have you laughing. If that doesn't convince, just watch the animation. That alone will keep you hooked.