Here are the features I like
1. Detects errors in HTML and the error messages are useful
3. Inspect mode – Pointing the mouse over the page highlights the code associated with that place and you can even modify the code.
The above 3 are the features I found most useful.
Others good features include
1. View DOM structure of an HTML
2. Measure Network activity – This is great to have if you make AJAX calls and you need to measure the response time.