Classic mode is awesome. I haven't played in quite awhile, but there were some good servers with ridiculous amounts of awesome mods.
It runs much faster than normal Minecraft, and some of the features like cuboid, permissions, multiple maps running on the same server, etc, still haven't been added back to the main version of the game.
Because they aren't part of the game - they're server plugins, and are all available for Bukkit by now (and have been since at least Beta).
Cuboid is part of WorldEdit, there's tons of permissions plugin (PermissionsEX seems to be the most mentioned, though in both positive and negative terms) and there are quite a few MultiWorld plugins for Bukkit nowadays.
It runs much faster than normal Minecraft, and some of the features like cuboid, permissions, multiple maps running on the same server, etc, still haven't been added back to the main version of the game.