The preview center rocked. It was really well done.
Out front, there was a huge (20 ft tall?) hour glass with sand running through it, counting down the opening to the park in 1999.
When you walked into the Preview Center, you were given a passport with about 5 pages, one for each island. You then progressed through a series of rooms, one for each island. You would get your passport stamped in each room. The rooms were done very well, all decked out in theming from that island. Concept art adorned the walls, there were models, figures of characters, etc. I remember the Jurassic Park room had a huge dented metal door that said "Raptor inside" and it would shake every couple seconds and scare the crap out of you. It sounds corny when I type this, but it was very well done.
In the last room, there was a huge physical model of the resort (including stuff that never was built, like the adjacent golf course and expanded production facilities). Once you toured it, you absolutely wanted to go. I wish I could go back. I remember all the concept art for the Lighthouse had the flame burning on top -- I wish that was never cut.
For a while, one of the Passports from the preview centre was hanging in one of the windows in the Port of Entry, but it has since been removed.