Just to be pedantic, Frank--the modes people have been talking about are diatonic (can be played on the white keys of a piano).

Peter T., you can still do tonic and dominant in any mode (except Locrian, I guess). But Bob Bolton and Barbara noted that in the Dorian mode, a lot of what moves the music is the alternation between the tonic and subtonic--for example, D minor and C major in the true Dorian mode, where the tonic is D.

What you can't do in most modes is combine tonic-and-dominant with the most important source of melodic tension in most post-1600 Western music: the leading tone (the note a half-step below the tonic) that Barbara emphasizes. (If you know this stuff already, Peter, I apologize--but I'm having fun.)

Thus in the Ionian mode (C major), the dominant chord contains B, the leading tone, which we expect to move toward the tonic C.

On the other hand, in the Aeolian mode (A natural minor), the dominant chord is E minor, which contains G, a whole step below the tonic. The melodic tendency for that G to lead to A is much less strong than if it were a G#. Hence the "modal" sound (I think).

Therefore a lot of music raises that G to a G# to make it a leading tone. You can leave the F alone, giving the harmonic minor scale much used in Ashkenazic Jewish music. (Someone in the blues thread is going to tell me I'm oversimplifying.) Or you can raise the F to an F#, avoiding the "Jewish third" between F and G#, giving the melodic minor scale, as in Greensleeves. Either way, I think you no longer have a modal feeling.

Of course, you can do the same thing in any minor scale, raising the 7th and possibly 6th degrees by a half step.

What I love is not the way the harmony goes to the subdominant but melodic phrases that end on the leading tone. Oh, not all the time.

(And yes, the Lydian mode has a leading tone, just like major scales. I'll bet the reason it didn't catch on the way Ionian did is that it doesn't have a perfect fourth.)