There are a lot of people who play fast and loose with some parts of the lore – and sometimes, that can be okay. On a personal level, there are quests which refer to parts of your backstory which you might want to alter a little; you might want to play a human death knight who has allied with the Forsaken (that is, rolled as an Undead, but story-wise, considered as being human when they were turned, not already a Forsaken), for example, and just gloss over those parts that disagree with your story. That could be interesting.
On the other hand, it’s my opinion that there’s only so much that you can fudge and alter for your character without it becoming a problem. While there are times you can do this, there are a great many more cases where sticking to the established lore, abilities, items works a lot better.
Let’s consider a spectrum.
There are those things that virtually all role-players alter – the most obvious being appearance. Although most people don’t venture too far from their character model in their descriptions, we likewise assume that not everyone really does look exactly the same. Also in this category, we have things like personality (which so obviously varies from person to person that it barely merits inclusion; but then, this is the point of this category), backstory, and so on. There are archetypal humans, gnomes, orcs, trolls, and so on, according to the lore sources – and obviously, nowhere near all characters are going to fit this archetype.
This category is quite sensible, as most of the changes are relatively minor (in the scheme of things), while granting a greater variety of role-play opportunities, and don’t affect or conflict with the actual gameplay mechanisms. There isn’t much more to say about this.
Venturing into original territory a little more are those things that are a part of the lore, and extrapolated to a new level. Back when Mana Tap was still a blood elf racial, there were a few blood elf players who extrapolated that something akin to Mana Tap could be used on other player-characters for a ‘hit’ of magic to the blood elf – or, in fact, that it could be used at all for this effect, causing blood elves to use it to feed their addictions. While there may be some disagreement over whether this is a reasonable extrapolation of the ability, there is a clear connecting point between these. Other examples of this might be those who use hearthstones for long-distance communication.
This can be used to great effect, but it must be noted that a lot of people probably won’t have the same interpretation as you, so some hasty re-telling may be in order if two different players just can’t agree on a reasonable extrapolation.
Finally, there are things completely invented for role-play purposes. This is the category that I feel the most caution ought to be exercised with, as it covers such a vast number of things, many of which I think are inadvisable or just plain Mary Sue-ish. An engineer who invents something akin to a functional walkie-talkie may be perfectly reasonable, but I have rather less faith in a warrior who role-plays having a giant sword that can suck the life-force out of enemies if he doesn’t actually have anything akin to this sort of thing. Sure, a lot of it comes down to game mechanics – if you want to role-play having a life-force sucking sword, how do you explain getting into a fight and getting your behind handed to you because what you’re actually wielding is a perfectly ordinary sword?
This one genuinely can affect gameplay mechanisms, which can make it awkward – but on the other hand, may do nothing more than provide new role-play opportunities, and provided there’s no lore that actually contradicts it, I don’t see why it can’t be used to great effect. Basically, when it comes to this one, just be sensible – if it outright contradicts a point of lore, actively interferes with some pre-existing part of gameplay, and/or doesn’t add anything substantial to role-play, I’d suggest just leaving it out and going with something different. If you do decide to go with it, keep it realistic, simple and with a reasonable explanation behind why it should exist.