Let me cast another vote for San Luis Obispo.
Granted, San Diego is literally my idea of perfect weather, but the expense and traffic have been mentioned a lot already. I went to college for four years at Cal Poly SLO and absolutely loved it. I've always sort of wanted to go back, and this thread is making me nostalgic to the point where I'm actually considering looking for a job there, since my current job is meaningless. My family lives here (Sacramento), but SLO is close enough to visit, and my best friend lives in San Diego, so that's not a bad in-between.
It was a long time ago that I went to school there (87-91), so it has definitely grown up a lot since then, but being a college town you still can't go anywhere without seeing a hot chick working in a store or riding her bike or walking around. Or dude if you're into that. There were always tons of artsy things going on, and the Farmer's Market that was mentioned has been going on every Thursday (I think?) for decades. Plenty of beaches nearby, with Avila being sort of the one most preferred by college students, but worst actual beach, and Pismo and Morro Bay better beaches but a little less crowded. There used to not be a lot of choice as far as shopping or movies etc. but a lot of that has changed. We used to have to drive 20-30 minutes to a real multiplex theater or a Software Etc./Babbages type store, but I'm pretty sure that has all been remedied by now.
The weather is slightly colder during the winter and slightly hotter during the summer than San Diego, but not anywhere near the extremes that Sacramento experiences. If it got over 100 in SLO that was rare. Plenty of 90s, but usually lower 90s. And I think someone earlier questioned there being a ton of fog, but that isn't really common in SLO itself. You might hit it if you want to drive to Morro Bay at night once in awhile, but you're typically far enough inland that you don't get most of it. Hell, I think we get more fog here in Sacramento due to the American River.
It is NOT going to have the diversity of culture and nightlife that a lot of those other cities have though. Nightlife back in the day was pretty much college students getting drunk, and I can't imagine that has changed a lot, but unless you need to be out several nights a week doing something new and fun, which I don't, that is enough for me. You will probably run out of unique things to do pretty quickly though if that's one of your biggest concerns. There isn't anything really close enough to get that variety either without hitting Santa Barbara (not sure that qualifies as culture though) or Los Angeles. The towns in a 30-mile radius are just as barren of culture, unless you like things like the quaintness of Morro Bay (I met Jack LaLane there once, apparently he lived there) or the Madonna Inn (if it is still standing).
It all depends on how much culture and nightlife and that "big city" feel matter to you I guess. If they don't, I think it has a good blend of weather, location, cost of living, easy traffic, and a constant influx of hot college kids that really make it attractive. Fuck, now I really want to move there.