Engineering has been my favorite profession in WoW, both in terms of its usefulness in the game, as well as its status as an awesome profession for roleplaying. Maybe it's just because I'm a huge fan of steampunk, but I find that those gadgets and funny things you can make with engineering have a certain style that goes beyond simple utility -- You just look at an engineer with his goggles, his mechanical mount, and maybe even some sort of robot or machine trailing along after him, and you immediately get the feeling that this is a character with character. No other profession can give you such a distinct characterization: you're not just a rogue, for example -- you're a scientist rogue!
In addition to that, most other professions feel like "crafting" jobs added on to the regular game, which they are -- they may give you better stats in one area or another but otherwise don't add many new abilities. Engineering, on the other hand, gives you a lot of special abilities and buttons to push, all of which can start to feel like a special sub-class for your character, underneath whatever class he or she already has.
In fact, as roleplayers, many of us play up our status as engineers as much or even more than our status as a hunter, warlock, rogue, or whatever. That engineering style is so persistent that it can define our characters more than anything else -- our own Palehoof practically defined this style in the column devoted to engineering that he used to write every week, before he lost his horns and his hooves in a bizzare scientific experiment (and decided thereafter to spend more time with his family). His commentaries on practical and theoretical engineering serve as excellent inspiration for all roleplayers who would call their characters engineers.
A War of Ideas and Explosions
As you probably know, gnomes and goblins have long been the two leading races when it comes to scientific exploration, and they tend to have two very different ideas about what the science of engineering is all about, and this has led to certain differences between the two branches of this profession. Gnomish engineering sees science as the exploration of the cutting edge of science and technology, with all the failures and mistakes that can take place with prototype experimental devices, while goblin engineering sees science as the best way to practically get things done -- usually well done, as in, from more fire and more explosions!
The argument between these two rarely escalates into violence as such, except of course to the extent that their work involves weapons of war. Gnomes may be on the side of the Alliance and goblins more or less neutral, with leanings towards the Horde, but ideologically engineers' disagreements have no faction. You may easily find gnomes who practice goblin engineering, and orcs who take up gnomish engineering -- it all depends on what an individual wants to get out of science. Anyone mainly driven to investigate scientific truths for their own sake is more likely to be a gnomish engineer, while anyone who sees science as a mere means to an end (usually an untimely end for one's enemies) is a goblin engineer, regardless of their actual race or affiliation.
Gnomes have long held a reputation for wanting to explore the farthest frontiers of knowledge, utterly unafraid of what that knowledge might cost them. In many ways, the gnomish technological mindset is what might have been on earth if people had discovered quantum physics in the middle of the steam age, about the time that books like Frankenstein, The Invisible Man and The Time Machine were being written. (And by the way, if you are at all familiar with these novels, then you'll probably agree that all the protagonists of these novels would have been gnomish engineers if they had existed in Azeroth.) During the 19th century, science was developing into the more rigorous discipline it is today, and many people were just starting to realize that inquisitiveness could lead to discoveries, and these discoveries could change everything about your society.
Gnomes represent a people who took the dream of limitless scientific discovery to the extreme and were rewarded for it. The magical world they lived in didn't present so many rigid rules to them the way ours did: they wondered if it was possible to create a multi-dimensional teleportation device, and sure enough, it could be done! Who would sit around dreaming up things like stem cell research and spreadsheet software when they could just shock people back to life, wear telescopic x-ray goggles and use limited invulnerability force field generators?
If you're roleplaying a "gnomish" engineer (even if he's an undead who absolutely hates gnomes), you should feel free to expound on any sort of meta-scientific mumbo jumbo that you can dream up on the spot (or carefully work through in your secret underground laboratory, if you have one). A gnomish engineer who isn't at least very curious about the mysteries of existence isn't really a gnomish engineer at all -- ideally he or she should have an intense passion for investigating such things.
If this passion for learning isn't your thing, and you just use engineering for its practical purposes, then goblin engineering is more up your long, dark, blast-stained alleyway. Excepting only those students of engineering who both restrain their curiosity and abhor violence (... Anyone? Are there any of these out there? Hello?...), all characters without that insatiable desire to know probably fit into the category of those with an insatiable desire to blow things up.
This isn't to say that a goblin engineer is necessarily obsessed with all sorts of destruction and pyromania (although that's certainly your most obvious option, if you wish it) -- you may only feel that engineering is a practical way to get your job done. Does your rogue feel very sneaky when it comes to backstabbing his enemies but cannot be bothered to practice his boring lockpicking skills? Why not throw away the delicate wires and just blast open locked doors and chests! Is your character sick and tired of having to come up for air all the time while investigating underwater ruins? Learning to make an underwater breathing helmet might be just the ticket -- your character might not need to understand the complicated scientific theory behind the device the way a gnomish engineer would -- he'd just want to follow the instructions, put the darn thing together, and use it!
Excepting of course those slightly insane and increasingly short-lived goblin engineers who are obsessed with explosives, your average goblin engineer might just be the type of guy or gal who wants to use fancy gadgets more than he or she wants to understand them. Being as your character likely has a need to destroy enemies, he or she probably considers explosives as an important tool as well. Where a gnomish engineer wouldn't be content with simple explosions and would instead invent a cosmic death ray to channel the latent nuclear power within every atom into a stream of white-blue death, a goblin engineer would give much more weight to whichever device makes a bigger bang.
In short, engineering isn't just a profession; it's an attitude. It's a way of thinking about the world you live in, either as a lifelong mystery to unravel, or as a tool to be exploited for whatever goals are most important to you. It's a style in itself, a class within your class, and while it does have its shortcomings, many people agree it provides a vast amount of entertainment and character, arguably more than any other profession in the game.