I always find it interesting when people on the internet point out that they can't understand the lyrics of a song. None of the people I know in real life have ever told me "I like this song, but man I wish I could understand what s/he is singing." Personally I've always though of the human voice as just another instrument. The only two things that come to mind right now that set it apart from most musical instruments are that each person has a unique voice, and that you can use words when you sing to express very specific ideas.
I personally actually find the possibility to convey very specific messages somewhat limiting in many cases; music without words can convey very abstract and not specific thoughts/feelings/atmospheres/(even stories) which are different from listener to listener and can change every time you hear the music. People can of course leave room for interpretation in their lyrics, but when that is not the case and they had something very specific in mind, they lock what they want people to get from the lyrics in place. That's not a bad thing in itself, but if the content of the lyrics is very uninspired, the words can really drag down a good composition. Once you've paid attention to bad lyrics in a specific song, it's hard to not hear them.
After all that I've just said, I'd like to point out that there are of course many songs that become a lot better once you understand the lyrics, and even a lot of songs that just wouldn't work on any level without the words.