- Bugs or lack of them

- Performance (you've been to those dog slow sites)

- Security issues (a category of bugs worth their own line)

- Dependencies - a 200kB theme really isn't 200kB if it requires 20 Javascript and CSS files packing another 4MB on.

- Known use - there's a sweet spot since you don't want to be the only one using a theme. You also don't want 90% of the other sites on the Internet to look exactly like yours.

- Open-source versus closed-source (regardless of price)

- Community engagement

- Author responsiveness