My 2 cents:
1. Is it robust - does it have out of the box support for catalog management. Can you easily import, price, inventory & manage items?
2. Is it fast - does it offer good performance, reliability and scale well.
3. Is it standards driven - CSS/easy to modify templates.
4. Is the developer base strong - can you extend it - if not, is it actively maintained & supported.
5. Security/compliance/patching - does the developer have the resources to fix issues?
6. Does it have search capabilities, parametric filtering, strong taxonomy configurations?
What is your store? Simple widgets with 4-5 bullet point configurations or is it items that people look for that match specific features/components/configurations? Do you track inventory? do you need to import supplier catalogs?