I've heard many mention that a trip to La Jolla is mandatory, if in Southern California. What's the big deal about it? What to do in La Jolla?

The big deal is it's a beautiful place! The beaches are beautiful, many with coves or rocks down to the water. Part of the "downtown" area sits on the beach so some of the restaurants have water views. There are nice shops & galleries to browse & the residential areas are quite beautiful. Go for breakfast or lunch to the Brockton Villa & have great french toast & a beautiful view. Then have a walk along the water across the street & watch the seals sunning themselves. Or go for a sunset drink at George's at the Cove.

The big deal is the geography. The beaches and waterfront park areas, together with the backdrop of the hills rising from the ocean covered with homes, is pretty.
There are a few good restaurants (George's is my favorite, as well) and some small shops in the walkable downtown area. But the area most people are referring to when they say 'go to LaJolla' isn't all that large. Don't expect South Beach Miami or Santa Barbara.

In a word - beautiful. We spent a week in Southern CA and found ourselves in La Jolla more than once. The Coast Toast at the Brocton Villa and a stroll along the coast afterwards is a must. We enjoyed both the "cove" and La Jolla shores.

Every post you've got so far is right. La Jolla is beautiful, casually swank, laid-back and upscale. There's no Hot Scene to speak of; you go there for the relaxed atmosphere and the breathtaking landscape. Good restaurants (love George's!!), little shops in the Village, a strolling town. Once in a while, you just need a dose of "tranquil beauty".