A flower-shaped lens hood comes with most ultra-wide lenses, and is useful for minimising ghosting and flare. In some cases, the hood is built into the lens.
Not all ultra-wide lenses have a thread for attaching filters. They're absent on the Samyang and Tokina 10-17mm ones, and can only be used at the longest zoom on the Sigma 8-16mm lens.
All the lenses featured in this group have sturdy metal mounting plates, rather than the plastic mounts that are fitted to some cheap lenses.
Ring-type ultrasonic autofocus is best - it offers very fast and near-silent operation, along with full-time manual override.
A bigger zoom range can be a bonus, but for ultra-wide lenses a short minimum focal length is the key factor. About 10mm is ideal.
Focus distance scale
This is an advantage because it enables you to manually set the hyperfocal distance, for maximum depth of field in landscape photography.