Seasons were really nice; they added a natural dynamism to the region of Unova. The idea behind seasons was that if you were to set your game aside for whatever reason and then on a whim decide to pick it back up a month or so later, it would seem completely different and that would encourage you to explore once again--I think that certainly worked. The only negative aspect I saw was that some events were tied to the changing of the seasons: the two biggest ones I remember were in B2W2, where you could challenge N only once per season, along with the Shadow Triad. The Shadow Triad, I felt, could have been challenged more frequently, whereas it made sense for N's to be seasonal because of his weather teams. In cases like that, along with the fact that some Pokémon are much harder, if not impossible, to obtain during certain seasons, makes me think that perhaps if the feature were to return it would be a bit more successful if the seasons passed more frequently--perhaps every two weeks rather than four?