You are forgetting the fact that Natsu is the protagonist, and those have ceratin... traits. For instance, they are able to overcome ANY difference in power, speed, wits, etc, with sheer willpower. That COULD make Natsu be considered an S-ranked mage, given the right circumstances.
And Gray, being the "Friendly childhood rival" would not accept the fact of being left behind by Natsu, so, again, given the right circumstances, Gray would rapidly catch up to Natsu, thus becoming S-class too.
I'm not sayin they are S-class yet, since I agree with goldb, they lack the expirience other mages show (About Juvia I'm not so sure, but then again, we haven't seen her fight seriously). But in my opinion, they could complete S-class missions easiliy and defeat certain S-class mages.Rest assured, they will be S-class in due time.
Now for another question... Speaking of the "tiers" these characters are in... What do you think about Lucy? I believe she has practically limitless potential, and given time and effort, she will become one of the strongest members of Fairy Tail. All she needs is a way to support her spirits and not let them do all the fighting (Don't tell me she can fight alongside her spirits, like she did with Loki against that creepy Raijinshuu guy). A REAL way to support them in combat, and after that she'll just become a stronger character. Let's face it, her spirits are quite something, and I recall reading during the first chapters, she said when a stellar spirit mage gets stronger, so do the spirits.
BTW, nice topic. Let's make it lively!