(Please keep the replies friendly. If you don’t like someones opinion, tell them in a polite manner and say why you would disagree.)
Essentially everything that was made using Bukkit’s API will, as far as I know, will work in Sponge. By work, I mean that those plugins can be rewritten for compatibility with Sponge’s API. Server users won’t be able to use existing Bukkit plugin jars with Sponge.
I think there is a lot more power behind the plugins now with this system. If you hook into forge directly along with the api you get even greater power.
This is a modding api on the backend of sponge. So hooking into that allows even greater changes.
With Bukkit API using reflection was practically the only way to get near internals without depending on actual files containing them.
Agreeing with coaster’s post. Since Sponge will not be completely built from the ground up and uses forge, possibly current FML developers can move work to Sponge (possibly allowing for huge flexibility considering how many FML mods are ‘out there’?)
This technically isn’t true, they will be able to use bukkit plugins on sponge, but only if pore ever becomes a thing.
Also depends what the plugin was accessing. I doubt pore will let plugins with NMS code work. And if it ever supports worldgen plugin I doubt it will be optimal.
That comment was made about a month ago. I didn’t know too much about Pore at that point.
Hello, @EpixPenguin! As a learning plugin developer myself, I have looked quite deep into this subject. The simple reality is, we wont exactly know until that time comes, but we can speculate what could happen, and what could be implemented into Sponge after it has been released. But my hopes are, simply, to have a better working interface for the plugins rather than Bukkit’s, and to see a more active community, that shares ideas and helps newcomers make their own custom plugins. Anything else can surely be added after, but with a strong community, anything can be possible!
As everyone said, it will work like a Bukkit Plugin but being made more easily 
I think we’ll actually have way more than bukkit, the new ECS looks very promising. (Hopefully I won’t have to spend hours messing around with NMS to get good AI’s this time). Though most of the core parts of bukkit will be there, just with improvements.
I wonder if there will be a way to stop hackers easily with Forge
I’m sure Anticheat, NCP, etc. Will all be ported over, and actually working this time. In 1.7 a lot of hacks came around due to Anticheat being extremely easy to bypass (Example: Spam get out of bad packets and then send a fly packet and anticheat won’t detect it), and NCP seemed to go down the drain when they messed up their event system, and this allowed for tons of bypasses to come out.
Because Sponge will have ForgeAPI available, my guess is Forge has a API that allows plugins to detect what Forge client mods are installed. Im sure a plugin developer will create a plugin that kicks players unless they dont have any forge client mods. as for Non forge client side mods, i got no idea how to stop them.
Another idea is a Forge mod creator creates a mod that stops you from joining the server unless you only have that mod, this mod will also detect for modified Jar signatures. Then server owners use the same method as mentioned above but instead of detecting no forge mods, it detects only that forge mod, if they dont have that forge mod it kicks them.
These are just ideas, not sure if they will be created. I maybe able to create the plugin myself, im not familier with SpongeAPI or ForgeAPI so not sure if it is possible