Engineering in Wrath of the Lich King is very different from what you've experienced previously. For example, Engineers used to be weighed down by trinkets, if they ever wanted to be able to use many of their gadgets. This system is slowly being replaced by a special type of "enchant" that allows an Engineer to consolidate.
Engineers are also beginning to focus on designing items that can be sold to other players, which adds another side to the profession.
There are currently no dropped Engineering schematics, although the plans for the two faction-specific mounts come from reaching exalted with the Horde Expedition or the Alliance Vanguard.
Cobalt Frag Bombs, introduced with the latest expansion, offer an incredibly cheap way to level your Engineering. This is perfect for players that are still leveling their characters, and can't fly around Northrend gathering whatever they please at efficient speeds.
These bombs are orange at 350, and go yellow at 375, and green around 380. The Volatile Blasting Triggers needed can be found in the Borean Tundra. in the Geyser Fields. There are plenty of little mechanical mobs that can be killed, looted, and then salvaging their parts, much the way a Miner can mine certain rocky corpses.
At this point, your bombs are yellow quality, and you learn two new recipes. The Bladed Pickaxe and the Hammer Pick are not only cheap ways to level, they are useful items that can be sold to other players.
The Bladed Pickaxe serves as both a mining pick and a skinning knife, which is perfect for gatherers who chose Mining and Skinning over Herbalism and Skinning, or the tracking nightmare that is Mining and Herbalism.
The Hammer Pick is ideal for Blacksmiths and Engineers who took Mining alongside their primary professions.
While this is not a leveling tool, the Belt-Clipped Spynoculars will save you some bag space and make you look more like a bonified spy.
This clips to your belt, and replaces your Ornate Spyglass, if you carried one. It allows you to see far into the distance, and it shows all of the locations of gas clouds on your mini-map. These two functions are also carried out by many engineering goggles, especially epics, but you may find yourself without an appropriate pair while leveling.
This stacks with the Eternal Belt Buckle made by Blacksmiths.
You will also learn the Flexweave Underlay, which allows you to turn any cloak into a Parachute Cloak by adding this "enchant." This does take the place of an enchant, so it will likely be more popular for casual players who farm, quest and do some dungeoning or light raiding, than it will be for people who love to push their stats up as high as possible.
Previously, if you wanted your cloak to give you a Slow Fall-like effect, you needed a separate cloak like the Parachute Cloak (30 second cooldown) or the Skyguard's Drape (30 minute cooldown). This new method not only allows you to wear any cloak you like, the effect is usable once every minute.
Both of the above items are yellow when trained at 380.
At this point, you can learn another belt "enchant." This does not stack with any other belt "enchant" that you can produce, although again, because the belt buckles do not count as enchants, you can have one of each.
The Personal Electromagnetic Pulse Generator will stun nearby mechanical entities for three seconds. This is its only function, and does not work on vehicles or seige weapons. It would be especially useful while questing through Northrend, or while farming mechanicals for parts in an area like the aforementioned Geyser Fields of the Borean Tundra.
This schematic is yellow when trained.
Also yellow at the time of training is the Hand-Mounted Pyro Rocket. This glove "enchant" will take the place of a traditional enchant or armor kit, but effectively gives you a ranged weapon that deals 1035 to 1265 damage, usable once per minute. It is also an instant cast.
Not only is this useful for classes that have trouble pulling from a distance, it is also a nice boost in overall damage. Place the rocket onto your gloves, put the icon into your bar, and all of a sudden you have a 1-1.2k burst of damage every minute. It is also useful for runners and pulling mobs out of the sky.
It is important to note that this item has no minimum range, and so can be fired by melee at point blank range, although it will reset your swing timer as well as activating the global cooldown.
On the positive side, it can crit!
This replaces the old Goblin Rocket Launcher, a trinket, that also gave 45 stamina and a 3 second stun effect. In essence, you are swapping out a low-level trinket and a glove enchant, for a slightly less powerful but very stylish, fun and useful "enchant," along with the latest and greatest trinket of your choice.
If rockets don't get your blood flowing, you could instead "enchant" your gloves with the Hyperspeed Accelerator. This does not activate the GCD, and awards you, upon use, 340 haste rating for 8 seconds every 2 minutes.
This will stack with other haste-increasing enchants, potions, food, and talents and abilities, such as is being discussed in the comments on Wowhead.
Both of these glove modifications are usable in arenas. Also, if you happen to be a Blacksmith-Engineer, these will stack with your Socket Gloves ability.
The Nitro Boosts can be trained at 405, yellow quality, and are used to modify your boots. Again, this takes the place of a regular enchant, but gives you an ability similar to Sprint, although it is only a 5 second duration with a 5 minute cooldown.
The speed boost is 150%, which is much greater than Sprint. This will take the place of a traditional enchant, but does allow the Engineer to wear any boots he or she pleases, rather than being stuck with Rocket Boots Xtreme or Rocket Boots Xtreme Lite.
You might consider making a pile of Scrapbot Construction Kits. For 10 Saronite Bars, you get a stack of 5 bots that will live for five minutes, and buy your junk. It will not repair for you, but can be summoned every 15 minutes. This is great for extended farming.
At 425, MOLL-E, the portable 5-minute mailbox usable once per day, is an excellent way to gain one skill point. You will be everyone's favorite raider when you walk in with MOLL-E and a stack of Field Repair Bot 110Gs.
While the cooldown isn't ideal, the fact that MOLL-E is a keepable item rather than a consumable is a huge plus, and this is something that Engineers have been wanting for a long time. Unfortunately, she doesn't currently fit in your Engineering bag, although I'm sure that will change.
The Gnomish X-Ray Specs, while they do actually replace your head piece, let you see everyone in their underwear. 'Nough said.
You will also be able to make a nifty frost resist helm, Mechanized Snow Goggles, which should be helpful for parts of Naxxramas.
You might also make a Global Thermal Sapper Charge, that will not only inflict AoE damage, it will also affect seige weaponry.
Epic goggles become available for you to create.
At a whopping 450 skill level, you will be able to create either the Mechano-hog or the Mekgineer's Chopper, provided that you are exalted with either the Horde Expedition or the Alliance Vanguard, which you should be by that point.
These cost a bunch to make, can be sold to non-Engineers for even more, and even have their own achievement, Get to the Choppa!.