Pachacamac is an easy half-day trip out of Lima. I took one of the dozens of minibuses that leave all the time from Avenida Grau. It dropped me off right at the site's entrance. There's a little museum on site, displaying some pretty Wari ceramics and also textile that has survived the ages due to the extremely dry climate here.
Its location indeed is one of the most remarkable things about Pachacamac - it's out there in the desert, within sight from the sea. The site is totally covered in sand. Archeologists are only slowly making their way to the many temples and other features below. Most of it is off limits to visitors, though the Temple of the Sun can be climbed and other parts like the North-South Street have explanatory boards. You can do a full circuit walk of the large terrain in about an hour (if you have a car, you can also drive).
Pachacamac was a ceremonial and pilgrimage site between 200 and 1535. It was used by different civilizations: the Lima, the Wari, the Ichma and the Inca. For me it is difficult to say if it merits WH status. Its most distinguishing fact is that it predominantly is a Wari site, a culture which is not represented on the List yet. But it's not the best place to see Wari remains - Pachacamac never was a central site for them, and the Inca added and altered a lot.
(P.S.: it might become part of the Qhapaq Nan WHS proposal, as this was one of the special places that the Inca built roads to)