i've lived in all 3 places. I worked in LA for a year, and my family lives in SD, and I now live in Irvine for work. I hate LA as a city. It's congested in traffic, it's dirty in air, the people are...just, not my type I guess. It does have it's good things - things to do, great places to eat, a lot of diversity.
Irvine is great. It's clean, low crime, convenient to LA, quiet. the OC is overrun by beach tourists and people from LA. A lot of the LA'ers grow up and start jobs and move to Irvine or the OC and well, as you've seen above, they're not totally my type of people. It gets too touristy and annoyingly conformist in some aspects - especially the clubbing in hermosa beach. I mean how often can you go to sharkey's or sangrias and still have a blast? the first 20 times might have been fun but come on...there's more than that (unfortunately my friends don't agree totally so i'm stuck still going sometimes)
San Diego might be more conservative, but man, the air is nice (it smells sweet in North County), the city is pretty clean, there are ample things to do, the beaches are nice. I like SD the most of any place in So Cal I've lived. It has its downsides too but of all three places, I like San Diego the most. Now if you're in college and/or under 25, SD isn't very fun - go for OC or LA. But if you're getting serious about settling down or perhaps being a little farther from the action yet close enough to go on the weekends, SD is a good choice. But trust me, SD is boring in terms of "exciting" things to do compared to LA and the OC.