Commute time (lower time = higher rent)
Travel time to shopping (lower time = higher rent)
This means that mixed use buildings are going to have higher rents due to this alone!
Household happiness, health, and wellbeing (higher values = higher rent)
Tax rate (lower tax = lower rent)
Side note: Children reduce the tax rate of the household! Currently, birth rates are controlled by random chance, but a fun mod might allow citizen happiness to affect the birth rate chance!
Highest household education level (higher education = higher rent)
Service coverage (police, healthcare, communication, education, garbage, entertainment, welfare) (higher coverage = higher rent)
Dwelling unit size (lot size * floors / units) (higher size = higher rent)
This means that zoning high density alone may not be enough to help. Consider zoning smaller lots to force smaller dwelling units, but be careful that units don't get too small, because this has a negative effect on happiness.
Land value (higher value = higher rent)
Urban blight (nearby crime, abandonment, pollution, homelessness) (more blight = lower rent)
Building level (higher level = higher rent)
Total net worth of the company is less than -$50,000
Estimated daily profit of the company is lower than the rent