Regarding Long Jing - the information given so far is correct. It's one of the teas you really need to pay attention to your sources for. There is sooo much variance there, and it's also not the most robust tea when it comes to storage. It is likely to change it's character just do to improper storage. Some teas don't have a huge difference between "fresh" and "not so fresh"... but with Long Jing, it's really noticeable. So if your supplier doesn't ensure that his stock is rotating quickly, you can end up with some pretty crappy tea. So, if you find a good supplier, stick with them, and make sure that they sell enough to keep thier stock fresh.
Also, depending on when the leaf was picked during the season, and how careful the tea makers were about it's oxidation period, the tea could be very smokey, or very light, very astringent, or very nutty... Check the leaf size for it's age, and check the colour for it's oxidation. A good Long Jing will have a lot of lighter coloured overtones, but generally be a emerald green ranging to a darker green. The dry leaves will be very uniform and thin. The dry leaves shoudl be very fragrant, and when drinking it, there should be a very complicated layering of flavours, so that you can easily discern many levels to the flavours. A slight smokey or nutty overtone should be the first impression, followed by the astringency of the tannins, and then finishing with a soft fruity sweetness. Overall the flavour should be very full and balanced, without leaning too heavily on any of those three components. Through multiple brewings you'll notice the flavours fall off in that order as well, losing the nuttiness/smokiness first, then the astringency and finally being left with a smooth and sweet tea, though it'll be rather weak by this point.
Anyhow, regarding Skye's comment about how to try out some teas without filling your cupboards with tins... That's just a symptom of the large retail distribution system that most US tea companies use. If you go to a smaller loose leaf tea shop, you can probably get small samples for free, or a very cheap price. Adagio sells little sample tins for all thier teas. Also, you could search out a coffee or tea house with a good selection of loose leaf teas and just try a cup of whatever tea you're interested in.
I'm not fishing for business, but I happen to be a tea salesman. I regularly give out free samples. My customers can ask for samples of any of my teas, and I'll send them enough for a cup or two of each tea, at no cost, trusting that they will find at least one tea that they can't live without. I specialize in high quality Chinese teas, so I happen to have Long Jing in my stock right now. The Long Jing I carry is a ZheJiang Long Jing, so if you're interested in that one I could send out a sample to anyone who's interested. As for XiHu Long Jing, I can get that one too, but don't have any in stock right now. One of the services my company does is to track down whatever wild or crazy tea request you might have, in person, in China. So far, we've managed to find everything we've been asked for. So, feel free to send me your questions! I love the tea sleuthing adventure!
my email: email@example.com
Hope that helps,