Hi R.-
We moved to Orange County a year ago from Northern Ca. My husband's job is in Santa Ana, (soon to be Irvine--they are moving), and we didn't know where to start looking! We looked ALL over--many of the communities that have been mentioned here: Lake Forest, Mission Viejo, Aliso Viejo, Anaheim Hills, Villa Park, Tustin, Irvine, and finally, Huntington Beach. I lived temporarily in Tustin and got to know that area & Irvine fairly well. I think what you really need to focus on is what kind of life you want. My take on it is that in Tustin/Irvine, the houses can be beautiful, but there really aren't much in the way of yards. A community we considered were the Columbus Square homes in Tustin. Here's the website: http://www.villagesofcolumbus.com/
All the homes are new (though now, there might already be some resales due to the economy), and the way the community is designed is really nice. The Square is in Tustin, but is districted for Irvine schools, which are better. The Grove is in Irvine, but has Tustin schools (still good, but not Irvine), so watch out for that distinction.
As for us, the cities themselves of Tustin and Irvine are both fairly newer, so have many strip type shopping centers, and parks, but no real "downtown" (although old Town Tustin is quaint, but not practical for shopping, more just a fun place to wander). We wanted somewhere with a bit more "character" and we love the beach, so we started looking in Huntington Beach, and were really surprised at what we found. There are many affordable homes here, with yards, no Mello Roos or HOA (do you know what that is? An extra tax on top of property tax or HOA dues), and the parks here are incredible, as well as the best Library & Children's Library west of the Mississippi. We're less than 10 min. to the ocean, and we've settled in a really lovely neighborhood with 3 parks in walking distance. Plus, the temperature here is more mild than inland. You actually notice a 10-15 degree difference. Although, coming from Texas, that might not matter! ;) Finally, even though we're at the coast, my husband's commute is only about 20-25 min., which isn't bad at all for what we're getting in return.
Good luck, I know moving is hectic--for our move we essentially moved twice, once into our apt. in Tustin, then again into our Huntington home--and I was 8 months pregnant! Oh--our pediatrician is in Tustin, Dr. Dawn Bruner ###-###-####. We've seen Dr. DangVu in the same practice also, and she's great too.
Once you get here--join your local MOMS Club, that will help you to meet other stay at home Moms and give you the lowdown on parks, have some playdates w/ other Moms & kids, etc. You can find your local chapter (Irvine has 3 or 4 chapters) at the following site: http://www.momsclub.org/
GOOD LUCK & welcome to the OC (sorry for the long rambling note--hope it helps!)