What can the Pyro return to sender using the compression blast?

You can reflect:

• Soldier Rockets (all types)
• Pyro Flares (all types)
• Demoman Grenades (all types)
• Demoman Sticky Bombs (Sticky Bomb Launcher/Scottish Resistance)†
• Huntsman arrows (which can headshot enemies)
• Sandman Baseballs
• Wrap Assassin Ornaments
• Crusader's Crossbow arrows
• Rescue Ranger bolts (will damage opposing buildings but not repair friendly ones)
• Mad Milk
• Jarate
• Level 3 Sentry Gun Rockets

When you reflect projectiles they are promoted to minicrits (if they weren't), change sides with your team and ownership to you (with the exception (†) of stickies). This means, however, that they will still do splash damage to you should they explode too close. As for stickies You can still push those off points and doors, however, or even push them below the demoman's feet.

You cannot reflect:

• Hitscan shots — anything which is a “bullet” and appears as streaks rather than projectiles
• Syringe Gun needles
• Bison and Pomson projectiles (you can reflect Cow Mangler beams, however.)