Subjunctive mood is something I learned from taking too many years of Latin and the occasional Prairie Home Companion show. In English we use this mood to describe things that are imaginary or conditional. Since that probably didn't help explain, here are a few examples:
I suggest that you leave. You haven't left, thus that you leave is in the present subjunctive mood.
I wish I were a woman. I am not a woman, therefore I were is past subjunctive.
If I were to have a hammer. I don't have a hammer, and were to have is more past tense than if I had, so it's pluperfect subjunctive.
Back to the Radiohead quote. I wish I was special is not past indicative (or normal past tense), because the singer is not special. If he were special, he wouldn't be saying that line, thus the was should have been were: I wish I were special. The fancy counterfactual conditional that I mentioned earlier is self-explanatory: It's a condition that goes against fact. He's not special, but he wishes he were special—it's contrary to his reality. Here are a few more examples:
Incorrect: If he did not kill the target, someone else did. (Past indicative)
Correct: If he had not killed the target, someone else would have. (Past subjunctive)
Incorrect: If it is dry, then he is not wet. (Past indicative)
Correct: If it were dry, then he would not be wet. (Past subjunctive)
Note that the lines I labeled as incorrect are not wrong by themselves; they are wrong if you meant to write in the subjunctive clause. Notice also that subjunctive implies that the statement goes against whatever the reality is. In the first example, it's implied that he killed the target. In the second one, it's implied that it is not dry.
Since this was finicky lesson, and you might not believe me, here is some further reading: Grammarist: Subjunctive, Wiki: Subjunctive, Wiki: Counterfactuals.