For me, the most important thing is that the products be useful enough to justify the trouble of collecting the ingredients and spending the time needed to make them. After that, I always like to see a broad range of possible effects (by broad I mean more than just "Potion of fire damage" vs. "Potion of frost damage").
After that, I like lots of different mechanisms involved in the process that behave in different ways and possibly affect ingredients differently, just like real chemistry. It's fun to need distillation apparatus of a certain quality, or with certain intermediate components or conditions needed to extract some compound from an ingredient. So a big part of the alchemy piece of the game becomes building, stocking, and maintaining a lab so that you can make the things you want.
Finally, I always like crafting processes to be part of a larger aspect of the game. This becomes more important to me the more grinding is required. Something like a store where NPCs can come to commission things for you to make, or buy from a selection of wares you've already produced sounds like fun to me. If there's some kind of reputation system in which you can become better known by making better products and then charge higher prices or get access to better NPC customers that would be even more fun for me.