Proper control of custom item metadata; Bukkit never allowed this, and using hacks (NMS access/reflection) was not a satisfactory solution, since custom item metadata could so easily be stripped by other plugins.
And maybe custom tile entities as first-class objects? (By which I mean attaching custom behaviour to server-side blocks without needing to write a load of custom plugin code to deal with scheduling/persistence etc.)
I am very exited for all the new sub api’s that will be implemented with the Sponge API.
I cannot wait to take advantages of those new loop holes which we have never had before and multi-threading!
I’m excited for WorldEdit and mcMMO in singleplayer.
Personally I am really excited for the Forge implementation. Love it. 
I’m excited for the unification between the client and the server.
And if the project gets there, Scala support.
I am happy that now Sponge dev team knows the pro and cons of Bukkit and now they are avoiding the mistakes and problems they made.
I am also happy that Sponge uses new markup language for the config files, finally config does not break from small mistakes.
I would like to see a plugin repository like Bukkit Dev but modern and safer.
The forums might look weir at first for me, I will hopefully get used to it. It has a lot of great features compared to XenForo.
Totally agree!
Especially with big configs, it is a pain!
I am really hyped for a sponge getting rid of the water between servers and mods! We can then empty out the water into the bucket and throw it away.
Java == Multi-Platform
Nuff said
I like that the community doesn’t feel so divided. Everyone has polled there resources together to make this happen. Topics are being addressed that were long shot down in the Bukkit community, where words like Forge and Suggestion were curse words. Felt like the open community had no say, whereas here everyone’s voice is being heard good or bad and progress is being made to put out a product that meets nearly everyone’s needs. We have to keep the Minecraft community whole through the dark times, and so far everyone is playing their part. Thank you all!
toi me sponge is someting to use develop plugins but no diffrent from bukkit