SG and FO are nothing like JM, Demo or Pacifica. I've ridden those places as well.
I try to hit JM on the weekends. Either there, Pacific or UCSC, where it's hard to get to during the week. Those are my favorite places. I haven't really ridden the crack, just checked it out from the bottom. We usually just do One Mile and Boyscout, but I haven't been there in a while. Mostly JM lately and I may go there on Saturday.
Parts of Downieville are similar, but you take a shuttle up so there's very little climbing. Approx 5000' descent in 15miles with maybe 500' of climbing. It's frickin' sweet. They do a race there in the middle of the summer too. I think this years race entries sold out in like 15 minutes.
FO has a loop that's mostly singletrack, but nothing technical. Just good beginner singletrack that's close to where I work. It's a great place to break in singletrack noobs. One loop is about 3.5 miles and 400' up and down. I usually do two loop plus a little extra to get over 5 miles and 1000' feet of climbing.
SG is a little better, with some rocky technical sections. The best part is the southern section nearest to the Hwy 9/35 intersection. There's another descent singletrack loop in the northern section of Longridge that you can connect to. There's also supposedly another good downhill section, but it's marked one-way only going up. Some people ride it, but there's usually too many rangers around for me to take the chance.
We just found out about Downieville shuttles starting up next week. We try to go a few times a year... more last year with Northstar being closed. We're just starting to talk about it and nothings 100% yet. Right now it looks like a day trip on 5/26. You're more than welcome to come along when we do go. That's how I got started. One of my first trips, I tagged along with the RideSFO guys. I'll post or PM you when I know for sure.
Can't wait until Northstar re-opens this year... 6/29 seems so far away.