The old ultimate better matched the hero's lore:

It emphasized the balance of light and darkness, as well as the concept of sacrifice for power. This is unique to Dazzle as a Shadow Priest.

 
The gameplay style has become less tactical:

Bad Juju required more skill and tactics from the player (for example, proper health management and timing of engagements), while the new ultimate makes Dazzle simpler but less original.

 
The new ultimate disrupts the hero's identity:

Dazzle no longer feels like a character who plays on the edge of healing and destruction. He has become a more "standard" hero with enhanced abilities.

 
The old ultimate gave Dazzle uniqueness in gameplay:

Cooldown reduction and ability enhancement through self-sacrifice made Dazzle an original hero who couldn’t easily be replaced by another support.

 
The community misses the unique mechanics of Bad Juju:

Many players remembered Dazzle because of his old ultimate. It allowed players to showcase their mastery of the hero and brought interesting decisions into the game.

 The Russian version of the petition is available here

This petition is addressed to the Dota 2 developers, Valve Corporation.