There are many things that I like about this episode, starting from the title, 'It's the Great Pumpkin, Sam Winchester', which is quite awesome with its length and gentle nod toward episode two, 'Are you there, God? It's me, Dean Winchester'. Then there's the writing, by Julie Siege, previously a story editor on the show, which can be both quite humorous (Dean shouting "Astronaut!" was priceless) and very dark and disturbing (the death scenes are gruesome, and the description of Samhain's rituals are bound to send more than one shiver down the proverbial spine). Finally then, the atmosphere, which was constantly gloomy and unnerving, with loads of gore and skeevy details, as Dean would put it, was so thick and scary that it is effectively one of the most frightening episodes of the entire season.
About the whole season, I have to add that it is really nice how the writers blended lighter, standalone episodes with arc-oriented ones, mixing them to concoct a very coherent, and yet extremely diverse, season, all focused on the different - and diverging - paths of Sam and Dean. Like in this episode, we not only see Sam using again his powers, against Dean (and the Angels' advise), but also the brothers' different reaction to the Angels themselves, and to their black-and-white kind of vision:: particularly interesting, and really important to understand what lies afoot, is Sam's flickering faith when he realizes that Heaven and Angels (Uriel, an amazing strait-laced, judgmental, nazi-like, resentful Angel) are very different from his ideal, the ideal he was praying to. Another interesting thing to note, in hindsight, is that the Angels'plan was clear from the very beginning of the season, that's saying not to actually prevent the breaking of the seals but to use and manipulate Dean and Sam in believing it.