An unnecessary limit
Unlike other Professions with vendor-obtainable recipes, Jewelcrafting is the only one with an artificial limit. Enchanters can purchase recipes in Dalaran with Dream Shards. Dream Shards are obtained from disenchanting high level (75+) blue items, which are abundant in Northrend through quests and crafted items. At worst, players can troll the Auction House for Dream Shards. The only limit then is money, which really isn't a problem in Northrend.
Leatherworkers purchase their patterns for Arctic Fur, which are slightly more expensive at the Auction House than Dream Shards, but Skinners (most Leatherworkers are commonly Skinners, as well) can skin them off most Northrend mobs. The point is, there's no limit to the accumulation of the currency for the reicpes. The further irony in that is that Wrath of the Lich King added only about 50 vendor-sold Leatherworking recipes, about 26 for Enchanting, and a whopping 102 for Jewelcrafting. You have no limit on acquiring the currency for the former two, but Jewelcrafters can get one token per day. One token per day to purchase over sixty recipes in Dalaran, all of which cost at least two tokens apiece.
Jewelcrafting can be incredibly frustrating, too, because it's the profession with the most recipes hidden away in the pockets of dungeon bosses. Actually, it's the only profession with recipes that drop only from dungeon bosses. Tailoring has shirt patterns that drop off dungeon mobs, but a Jewelcrafter looking to amass an extensive repertoire of gem designs must defeat ten dungeon bosses, seven of them at Heroic difficulty.
Stone Keeper's Shard shortage
It is also the only Profession that requires the expenditure of Stone Keeper's Shards, putting Jewelcrafters at a disadvantage. While others amass their 300 shards for the Black War Mammoth, Jewelcrafters need to spend -- if my spotty math is correct -- 216 shards to get all fifteen designs. Not only does Jewelcrafting get recipes slower from Dalaran compared to other Professions, Jewelcrafters also get other items from Wintergrasp at a much later time if they choose to purchase designs first.
Unlike most Professions, where most patterns are trained, Jewelcrafting has over a hundred recipes that knucklehead Timothy Jones and his pals won't be able to teach you. It requires the most research because if you don't know all the sources of the designs, you'll miss out on a lot of cool gems. It's brainless to ask a Blacksmith if he or she knows a particular plan because almost every Northrend Blacksmithing recipe is trained, including all the epic ones. All the craftable frost resist epic gear from Tailoring and Blacksmithing are trained. The Leatherworking ones are bought... but with Borean Leather. Seriously, they should've just given it away for free. The Jewelcrafting frost resist amulet and ring? Designs. Bought with tokens. Ugh.
When you ask a Jewelcrafter if he or she knows how to cut a particular gem, it will depend on whether they're fastidious with their dailies, chosen the right design to purchase, spent Stone Keeper's Shards on them, or farmed dungeons. It's not easy. It's inane. Ask a Tailor or a Blacksmith, and the only question is their level of skill, and whether or not they're thrifty with training.
One other problem with Jewelcrafting is that, despite the good upgrades through the leveling experience, there just aren't too many socketed items. On the other hand, most gear can be enchanted, making the demand for enchants more widespread than the need for gems.
Jewelry would've been an excellent avenue for profit, except that the designs for the epic amulets and rings cost Dalaran Jewelcrafter's Tokens. Six of them. It takes almost a week of dailies (can't miss one!) just to get the design. In order to actually make one, it takes four Dragon's Eyes, too. Dragon's Eyes are special prismatic gems only available through Tiffany Cartier for -- you guessed it -- a Dalaran Jewelcrafter's Token. If a Jewelcrafter wanted to craft a Titanium Spellshock Ring for herself, for example, she'll need ten days to do it all by herself. Of course, you could buy the Dragon's Eyes, but considering Jewelcrafters only get one a day, it can get pretty expensive.
Again, the trouble is the resource. It's bought with the same currency that's used for designs. A player opting to purchase Dragon's Eyes sacrifices saving up for a design. It's the same difficult choice that Jewelcrafters are forced to make when spending Stone Keeper's Shards. It's the worst-designed Profession in the game and has been for a while. WoW Insider' Dan O'Halloran dumped Jewelcrafting after getting it to 425 in favor of the easier and more profitable Skinning. With so many things in Wrath made so easy and much more fun, I don't understand how Jewelcrafting can be so flawed and frustrating. Ironically, it's the one aspect of the game that needs a lot more polish.