The new whirlwinds in ACZ are sweet. Yep. It's true.
I love the extra routes and exploration, giving the game a ton of replay value.
Despite the dense leveldesign being hardly designed for going fast, there's almost always a way TO go fast even in the slower sections, usually thanks to the speed thok or slopes allowing for clever tricks. Never a dull moment.
Super Sonic. ...it's Super Sonic.
The music. It really gives the game it's own identity and an edge over other fangames.
Online. With extra gamemodes on top of Co-op, which is just going above and beyond.
And of course, it's customization. Addons, options&console commands, playstyles. There's reasons SRB2 lasted as long as it did. Mixing this with online is at least half of it.