Mostly the nicotine, but with very low nic, you can get more TH if you use high PG.
As for TH... there's an additive called Flash, which contains a chemical called "piperine", which is from black pepper -- it's not as hot as capsicum, more tingly than hot -- I use Flash in everything I vape, to get more TH, since I already vape very high PG (85%), but can't handle much nicotine (currently 6mg, down from 10mg).
Andria