I just did the opposite move, from SF to NYC.
The Mexican food in SF is very specific - Burritos. Burritos are as much Californian food as they are Mexican. You'll be hard pressed to get a decent burrito in most of Mexico. You'll also be hard pressed to find a decent burrito outside of SF once you've lived there.
The burritos in NYC are a joke. However, tacos in NYC are fine. The mexican people in California are usually from California... home of the burrito. All the mexicans I've met in NYC are from D.F - where there are no burritos to be found, but there are tacos. You can get nopales with your taco in NYC, it's hard to get them in SF. Zed's right though... in general there's better mexican food in SF. However, NYC definitely is not the worst. Try getting mexican food in Wisconsin, for example.
The weather in SF is weird. It's kinda cold most of the time. When I was a kid in the midwest I used to watch skateboard videos featuring SF. I'd mimic the guys who wore a winter hat and a t-shirt. I was determined to be as rad as they were. My head got so hot, I couldn't stand it. I couldn't figure out how those dudes could do it. It all came together when I moved to SF. It's the only place in the world where it makes sense to leave the house in a winter hat and a t-shirt.
The weather in NYC is balls this summer, but I kind of like having an excuse to wear shorts for the first time in 5 years.
NYC is indeed dirtier overall. However, if Zed really wants to know where the part of San Francisco that makes god weep, it's 6th and Market. Human poo, needles, meth tweakers, etc. The lower tenderloin up the hill from that intersection is a also a horror movie.
Regarding sketchiness, the sketchy parts of SF are actually sketchy. NYC, not so much. I've been mugged twice in SF. Once right in front of my apartment. My roommate was beat up by three dudes for 18 dollars. The hells angels dude got mowed down right over where I used to live. 21st street is the dividing line between the Nortenos and Sorrenos... 2 guys got shot on the street in front of my startup, while I was at work. Note that this was Bryant street and 21st in the Mission... where tons of hipsters and software people live...in 2006. There are constant gunshots in the projects next to Hayes Valley, right up the street from everyone's favorite coffee shop, Blue Bottle. I moved to a "nicer" neighborhood, Cole Valley, and within months my house got broken into and my TV was stolen by a crackhead. Market street after midnight (or after noon, depending on the day) is like 28 days later or similar apocalyptic zombie movie.. and so forth. Don't get me started on Oakland.
In contrast, New York is so safe these days it's a joke. You have to try pretty hard to get to a bad neighborhood in NYC these days. I never thought I'd "go soft" by moving to New York, but I'm not looking over my shoulder for tweakers and thugs anywhere in manhattan. I left my bike locked up on the street in Brooklyn and nobody ganked my seat like they did in Berkeley. Etc.
Regarding tech, I haven't been here too long. I know Zed hates ruby, but we had a nycrb meetup at work, and it was much better than the ruby meetups I'd been to in SF. The people were really cool. I also thought the "lame" project email he received sounded like a fun project, and I work for a fashion company, so maybe I'm not as cool as Zed. I do miss TechShop in Menlo Park already, as well as all the South Bay weirdos who build robots and actually design circuits and work on network routing protocols and stuff like that. There doesn't seem to be an equivalent to that out here, but maybe I just haven't found it.