After seeing LEGO Harry Potter: Years 1-4 in action for the first time, two things stand out to me. First, the game is really visually appealing. I know LEGO games have always used cute characters and bright colors as a whole, but this one has some great set pieces and detail. Diagon Alley is popping with color and twisted buildings; there are the familiar paintings on the walls of Hogwarts, but they've been LEGO-ized; and Hogwarts looks pretty slick as you run through its stone corridors and assemble LEGO suits of armor. Second, LEGO Harry Potter feels more like a traditional game than any of the LEGO games I've played before.
Now, that's not a subtle shot at the Star Wars, Batman, or Indiana Jones -- games that have paved the way for this title. It's just pointing out how much this format has evolved. Back when you'd put LEGO Star Wars in, you could just jump into whatever episode of the movie you wanted. Batman amended this a bit because it was an original story, but you still had the large overworld where you'd run around with little direction as to where things were and why they were there.
That isn't how it works in LEGO Potter; you're going to play through the events of Harry's first four years at Hogwarts, covering the events of Sorcerer's Stone, Chamber of Secrets, Prisoner of Azkaban, and Goblet of Fire. As you do, you're going to mature just like the young wizard. You'll need to go to classes to learn spells, those spells will get more and more powerful as you keep studying, and you'll gain access to otherwise inaccessible parts of Hogwarts as the years go by.
Basically, you're playing Harry's life... but it's all told via LEGO blocks.