Most cities have many but one major, top-of-the-mind symbol. This is the one for Barcelona.
The masterpiece of Gaudi is probably one of the few structures ever created by a man that is strikingly awesome and fearsome; at the same time and to the same degree. Only, it's not created by a man. Gaudi transformed an entire city into what it is today, but couldn't pull through long enough to see Sagrada Familia completed, which most likely makes the whole thing even creepier.
All and all; Sagrada Familia is like some renaissance novel bursting with heavy symbolism and subtext waiting to be deciphered... and completed.
Be ready to spend some time circling around each sector of this daunting structure and observe its details; but probably a bit more to decide whether you are amazed or intimidated by it.