Everything maters as a package, but I guess the most important bits would be: bike fit; pedals; the hoods on you brake levers if you use drop style bars (converting to drops is you don't have them already); gearing for the task of touring vs your city bike uses; gloves; shoes/sandles; some kind of cycle garments like shorts with a padded saddle area; fenders if it is wet; hydration; shock absorbtion system doesn't have to be a shock, but tires and other stuff already mentioned...; quiet panier system that doesn't rattle all day; and a nice soft helmet to land on if you fall off.