I live in Astoria and I'm not crazy about it. Please take this description with a grain of salt -- no neighborhood is perfect, and it seems that most people love it here, but you might as well have a balanced view, right? A lot of these complaints are probably applicable more widely to living in the outer boroughs as opposed to Manhattan or the suburbs.. but I've never lived elsewhere in the outer boroughs, so this is the best I can do. Sorry this wound up so long...
If you're going to live in Astoria, I would strongly encourage you to live near one of the main drags -- Ditmars, 30th Ave., Broadway, and Steinway. Ditmars is considered by many to be the nicest, and Steinway the grittiest and most congested, but your mileage may vary. You also want to live near a subway -- that means living near 31st St. (N/W trains) or Steinway (E/V/R/G trains). (Don't live _too_ close to 31st St., though, because the train is elevated and thus quite loud -- a distance of 3 blocks or so is probably ideal.)
Astoria is a big place, and there's a lot of housing that's nowhere near anything, so be cognizant of that. If you're going to the hassle and expense of living within the city limits, then at least do yourself a favor and live near stuff. There's lots of new development down toward the park and waterfront, but don't get suckered in; the inconvenience isn't worth it.
Convenience to the city:
The N/W is probably Astoria's "main" subway line. Astoria is quite close to the city, and would be a really easy commute if the N/W wasn't so terrible. I seem to recall reading that the N/W has fewer scheduled trains in rush hour than any other line, and if that's true, I wouldn't be surprised -- the waits for the trains are long, and as a result, the trains get very crowded. I don't usually take the trains during off-peak times, but when I have, oh boy. I once waited 40 minutes for a train on a weekend afternoon.
If you work, say, at 59th and Lex, the commute's still pretty nice. I work downtown, though, and my commute's over an hour each way -- so if you know where you're going to be working, that may be worth considering.
Getting around the neighborhood:
Like I said, Astoria is a big place, and there's really no place you could live that's near everything. There are two subway lines, but most people live near one or the other, so getting around can be kind of annoying. Say you live near Ditmars/31st and you want to go to a movie on a Saturday night. It's too far to walk, but taking the subway there will take forever, because the trains come so infrequently, and even once you get there, you'll have another 7 or 8 blocks to walk. That's like 2 miles, and it could take you an hour to get there. So I tend to drive places in the neighborhood, but driving around here is enough to give you an ulcer, and parking is even worse.
As far as shopping goes, Ditmars, 30th and Broadway are mainly restaurants, cafes, and little stores. (There are some really great restaurants here which is one of the great things about the area.) Down towards Steinway, there's more of that stuff, and also some chain stores like the Gap. There's a UA multiplex, which is nice to have in the neighborhood, although it's a bit out of the way. There's no real bookstore in Astoria although I think there's a dinky little one on Broadway that barely counts. (There's regular lamenting about the lack of a B&N or similar store in the neighborhood on Astorians.com.)
The supermarkets in convenient locations tend to be not so hot. The closest one to where I live, the Key Food on 31st St., isn't terrible but it's rather small. Lately I've been going to the Best Yet on 37th St. between 19th and 20th Aves, which is decent. That's really in the middle of nowhere, though, so it's probably out of the question if you don't drive. There is also a Pathmark and a Stop & Shop on Northern Blvd., along with some other big chains like Best Buy and Home Depot -- but the Northern Blvd. area is probably not even considered Astoria and is hard to get to without a car.
This matters more to some people than others, but it's hard to deny that Astoria can be a dirty, ugly, noisy place. The elevated train above 31st St. is hideous and extremely loud. It's not at all uncommon to see trash in the streets, and graffiti is a serious problem in the neighborhood.
Astoria is fairly safe. Some areas are safer than others -- stay within a few blocks of the subway and you're generally OK. Some of the areas that real estate agents tell you are part of Astoria really aren't, and some of those areas are pretty sketchy, so pay attention to your surroundings. There's certainly not a lack of crime; there have been a bunch of burglaries and crimes of that sort discussed on Astorians recently, and there have been a couple of well-publicized violent incidents in the neighborhood in the past year, but it is NYC, after all.