Adequate sunshine, watering, nutrients, weeding, pest and disease control. I hear that they like certain types of music too.

Photosynthesis. aka more sun or longer exposure to it.

you can change the mix to control moisture, you can somewhat improve on humidity, but lighting is the more difficult requirement to meet, since most dwellings are static and cannot be modified easily. So better design of supplemental lighting can help a lot. Then people will be more willing to put up lights that do not look industrial or glaring and fit into living décor.

also wicking watering systems, like self-watering pots (Lechuza comes to mind) used with proper mixes can make taking care of plants less of a chore (if you are too busy, go away often or have too many :).

all of the above assumes that people already know plants optimal requiremnets for growth: which actually is not true for most part :)...beginners look at labels which very often give fuzzy or even incorrect information. so..drum roll!!...educating people about plants would be #1 enhancer! LOL