Well first of all it is the sound itself. Now I know that that seems self-referential because what is there in music apart from sound? I suppose what I mean is that there is a richness of texture and a really good sense of balance between the bass line, the melody and the inner parts. It is constantly inventive.
Digging a bit deeper I would say that the use of the orchestra is a constant joy. Just three examples out of many.
- the use of bassoons in the recitatives in Zoroastre
- the low sustained notes in the horns in the musette in the last part of Les Fêtes d'Hébé
- and my favourite - the totally mad oboe parts in Platée representing the pond life

Then we have the ensembles. Rameau is surely one of the great masters of the operatic ensemble - nobody came near him in my opinion until Mozart, and if one is talking about ensembles involving soloists and chorus then one has to look into the 19th century for anything as impressive as, say, the end of the first act of Platée or parts of Act 4 of Les Boreades
(although I must say that I find this too fast - Gardiner judges the tempo much more effectively in my view)
But finally there is the dance music. Surely Rameau must be counted as one of the great masters of theatrical dance music - in my view his only competitors are Tchaikovsky and Stravinsky, and much as I adore both them I think that I would have to hand the gold medal to Rameau. The variety is astonishing - at one time elegantly languid - at others infectiously foot tapping. Of course it is the dance that is the biggest problem to deal with in modern productions. I don't believe that there is any point in trying to recreate the original steps, but I do sometimes feel that modern choreographers must have a tin ear, given the astonishing gulf between the sense of movement in the music and what appears on the stage. The dancing in the performances of Les Boreades above is a case in point - it seems far too busy and fussy, as if trying to overcompensate for what the choreographer doesn't hear in the music.
Rant over!
So lets finishwith the wonderful contredanse that ends Les Boréades. I don't normally like clapping to live music but you can see why the audience wanted to join in.