Ever since my main Bloodshrike on Demon Soul started out, I have been in love with engineering. I dabbled a little bit with mining, skinning, herbalism as secondary professions to his Alchemy, but once I realized the true power of engineering, I made almost every toon after that have Engineering as one of their professions.
Engineering does 2 things.
One, it gives you limited access to abilities other professions have, if only for a limited amount of time, with a cooldown.
Two, it gives you explosives! I had been seduced to WoW from Diablo 2, where I had developed a build for characters to give any class Zeal, which was a multi-hit or AOE (crap, I can't even remember what AOE stands for anymore, whatevers, it hits any monster standing close to you) build.
I figured out that bombs would shave off at least 10% of a monsters hit points in one blast, more if you powerleveled engineering to the max whenever you hit the level caps to advance the profession.
Of course I'm not going to rage-quit over this, but it does seem like they neglected a major portion of both specialities of engineering. Goblin engineering is about bombs, they go boom! Gnomish engineering is about trinkets, they go snap-fizzle-zap-aaaaargh aaargh it burns!
Basically, we got a modified mote extractor in the Electrostatic Condenser, but it only works for gatherer/engineers, not just any engineer.
The Dragonling is a generic trinket that any class can use since it adds mastery, and the combat pet bonus aspect of it is pretty nice.
The Bolt Gun is a useful interupt and damage dealer, but it's single target only and requires 1 Handful of Obsidium Bolts each time it's fired.
The belt upgrades are awesome,you might want to put them on different belts to keep in your inventory. Target Dummy, Nigh-Invulnerability Belt, and the Gnomish Cloaking Device.
The glove enchants are ehhhh, again being ideas from Wrath. Armor, Runic Healing Injector, Runic Mana Injector, moar Intellect/spellpower, and a combo of the Gnomish Lightning Generator Trinket/Hand-Mounted Pyro Rocket in the Tazik Shocker. I'm not much of a fan of tying up a glove enchant on an average of 50% more healing/mana from potions though.
The Loot-a-rang is awesome, being an original idea that could definitely be useful if you kill an enemy that's fleeing, or on a hillside. I love the fun items in engineering.
Bigger Bags - Toolbox and Tacklebox. Really? There's people out there that go fishing enough to warrant needing a 36 slot bag for it? I just hate being forced to make 3-4 of these to level up engineering, because all other engineers have to make 3-4 of these as well, making them useless to sell on the AH. I only usually need 1 engineering bag with me, and maybe 1 in the bank to use for extra mats or bombs.
Scopes - Hit rating and Haste. Heh, just glanced at the Safety Catch buyout price in Wowhead, which said it was about 2 gold. That's 100 gold or so worth of mats that you're just giving away. I hate being MADE to make stuff that I'll never use. I probably won't be able to use all the scopes I've made on my hunter, not to mention my Rogue/Warrior/Paladin/Death Knight that soon will be maxing engineering as well. Granted, I should probably be just selling the mats right now, and wait until prices come down before maxing professions, I'm just kinda bitter that you're forced into making extra stuff for no benefit. Remember the Gnomish Army Knife? An awesome item that should have sold well, but made worthless by making it the most economical way of skilling up engineering.
Guns and Crossbows - Throat Needler and Volatile Thunderstick. Sell them at a loss, yay!
Pets - Actually, I might take back what I said about the toolboxes and guns. If you make sure to visit your appropriate specialization trainer at 475, the 2 new pets might actually give you a profit (it looks like they stop giving skillups by 500 though). From Gnomish trainers - the mechanical rabbit. From Goblin trainers - the Fel REAVER!!
Food and lures - Even though Kaliopes guide says to do the lures from 510-521, I'm looking at the guild achievement of putting out 5,000 feasts or whatever, and thinking that the BBQ might be a better option.
edit - Oh yeah, remembered later that AOE means Area of Effect.