
The scientific method can be very helpful here. Speculate on what feature(s) people care most about (a hypothesis). Present a choice between products / services that have and don't have those features (an experiment).
If you're creative enough you can come up with a large enough list of what it might be that you feel overwhelmed trying to present them all. In In that case do a binary search. For example, you can find the most important feature of 8 features using only 6 products (A-F) and 3 trials:
A(1,2,3,4) > B(5,6,7,8) C(1,2) < D(3,4) E(3) > F(4) Turns out people really like feature 3 even if they can't explain why.
If you're wondering how you present a product with only one feature realize what this really means is that there is only one feature of the compared products that is different.