Good icon sets share a few principles: consistent style, clear meaning and usable contrast.
Consistency
Keep stroke width, corner radius and size (e.g. 24×24) the same across the set. Icons should feel like one family.
Clarity
Prefer simple, recognisable shapes. Avoid packing too much detail into small sizes; test at 16px and 24px.
Accessibility
Ensure sufficient contrast (WCAG 2.1). Don’t rely on colour alone to convey meaning; use shape and label as well.
For more, see our Icon Guidelines and browse Icon Packs for inspiration.