This review contains mild and vague spoilers, but not specific plot/ending details.
Nope! This is not the shoujo anime you're looking for. Not unless you like over-dramatic, contrived storylines, thin and uninteresting characters, and more sap than a Canadian maple tree.
One day in high school, Naho receives a letter from herself ten years in the future. It accurately describes everything that will happen that day, including the arrival of a new transfer student from Tokyo named Kakeru. (Get ready to hear "Kock-ay-ROO" over and over again. You'll be hearing it a lot.) The letter says that Kakeru is no longer with them in the future, and Naho should save him.
The problems begin immediately; despite accepting that the letter is real, Naho ignores its advice the first day, and things go predictably wrong.
And really, Naho is the biggest problem out of the many issues which plague this show. There's no way to sugar-coat this, I'll just say it outright: Naho is a deeply stupid person. She is unobservant, socially awkward, and frequently makes terrible decisions. Most of the tension in the plot stems from her ineptitude, and she's rarely proactive--she either passively does what the letter tells her to do, or passively fails to take action, rarely making her own decisions.
There's an entire friend group in this show, and that's one of the few things I'll praise about it; they feel pretty natural together, and they're one of the more convincing friend-unit depictions I've seen in an anime. With that said, none of them have any real depth either. The most interesting of the bunch is Suwa, the odd man out in Orange's love triangle; he has a bit of pathos and courage, but we don't get to explore him in depth.
The series is mainly about Kakeru. Everything revolves around changing the future by helping him throughout the entire run of the series. But because he's not in on the central conceit--he doesn't know about the time traveling letters--he's a passive player in his own drama. And he's so defined by his own depression and regrets that there isn't room for anything else interesting. It's kind of hard to see why Naho and her friends like him, because he's kind of a quiet, personalty-less bummer most of the time.
So we have a situation where the protagonist is an idiot, her love interest is a passive, depressed bore, and the plot is based on a contrived cliche.
And it never really gets better. There's a lot of drama over banal things, like falling down in a relay race, or obviously contrived and ill-fated love triangles. There are emotional scenes where Kakeru's friends try to get it through his thick head how much he means to them, even though we as viewers never understand why he matters so much to them anyway.
At least it all looks nice. The animation is pretty good, and I like the character designs. The dynamics between some of the side characters are enjoyable, even though they're shallow; Hagita and Azu are amusing and provide some believable brevity to an otherwise glum and overwrought show. The time travel mechanics avoid the paradox problem, even though the answer to how the letters were sent is unsatisfying and goofy. The music and voice acting are fine.
But the writing is just too terrible for the studio to salvage much.
I'm not usually a fan of time travel, but that's not the biggest issue. Here's the bottom line: When you try to make a character-based drama, it's kind of important to make sure the characters don't suck. That's where Orange fails. I don't want to spend any time with these people. They're boring and dumb.