well i guess i should ask for the criteria first before asking questions or criticizing.
i wouldn't be surprised to learn that those two are more popular than tristan, but as far as its importance to music history, it seems to me that it might challenge any piece on this list. i guess it depends how you balance popularity with historical influence/importance and originality/quality as the criteria.
also, you mentioned that you did consider standalone pieces, after all, which means that "prelude to the afternoon of a fawn" would be eligible... is that right?
also, this is only for orchestral preludes and overtures, right? i think maybe you should specify that somewhere.
The list is based primarily on popularity and on esteem among musicians and students of classical music, with historical importance considered secondarily. It occurred to me a couple hours after I posted that my description of 1812 and William Tell was one-sided; they're popular, but are generally considered lightweight compared to most of the other highly ranked works.
Don-Alexei is right about prelude to the afternoon of a fawn. It's not really a prelude in the sense meant by this list; it's a symphonic (or tone) poem. It's on the tone poems list. The stand alone pieces I had in mind are mostly concert overtures. Admittedly, sometimes there's a fine line between a tone poem and a concert overture. Tchaikovsky's Romeo and Juliet is a work that could have gone either way. I have given some consideration to merging this list with the tone poems list and dropping the overtures that are fragments of operas, but right now I don't plan to that.
These are orchestral overtures and preludes only. Overtures are generally orchestral; I don't know of any that aren't. So we didn't need to specify that the overtures were orchestral only, and in the context of "overtures and preludes" we thought it was understood that we meant preludes that are like overtures, and that we weren't referring to something like Chopin's Preludes.