The concept of blasphemy is one that's common to the Abrahamic faiths, but is not widely found in other religions.
For many religions, certain words are never used, because it's considered blasphemous to do so. In some orthodox branches of Judaism, one is not permitted to write the name of God - if you're jotting down His name, you might write it as G-d, to avoid being seen as blasphemous. For Catholics, the sin of blasphemy includes blasphemy against the Holy Spirit -- the questioning of whether the actions of the Holy Spirit might be attributed to some other being or entity.
Some groups take blasphemy to an even more strict level. For example, in Islam, it's seen as disrespectful to draw a picture of the prophet Mohammed, and you could easily find yourself under a fatwa if you scribble out a cartoon with his image. Certain fundamentalist Christian groups see the celebration of any religious holiday with secular aspects as blasphemous -- colored eggs at Easter, or Santa Claus during Christmas would fall under this heading.
However, it's important to note that if you're not part of a religion, it's rather unreasonable for its members to hold you to their standards. In other words, Jews don't expect non-Jews to never write out the word "God," because it's a rule for them, that's found in their holy writings.
The dictionary defines blasphemy as disrespect - or at the very least, irreverence - towards something sacred or holy. Much like sin and obscenity, disrespect is typically in the eye of the beholder. In the Abrahamic religions, the criteria are pretty well established as part of doctrine.
In many Pagan religions, however, the deities are not seen as stern taskmasters, or angry old men who rule through fear rather than love. In fact, many Pagan gods and goddesses are a lot of fun -- they are often viewed as having a bit of a sense of humor, and not concerning themselves overmuch with the day to day activities of their worshippers, unless we specifically address them.
So here's the question for you -- do you think the goddess of your tradition finds it disrespectful for you to say "oh my goddess" when you drop a candle? Do you think that it so enrages her that she's going to stop what she's doing and somehow make you suffer? Or do you think maybe she's having a little giggle over the whole thing, and then going on about her business?
You're going to encounter fundamentalists in every religion -- and that includes Paganism. Don't let a negative experience with one of them color your entire perception of Pagan religion. You'll meet far more people who believe that the gods have a sense of humor, and that they don't especially care if you blurt out "oh my goddess" when you're about to set your ritual robe on fire. Honor your deities the way your heart calls you to do, and don't let anyone bully you about it.