TLDR is very difficult, this thing is huge. It's like asking for a tldr of what is the internet:) I'll let CryptAxe's response handle that one.

Is it an alt-coin market + plug-in framework?

Sort of but not exactly. It does implement some different coins in order to allow them to share their features with eachother. It is also a platform for business to build project on top of. For instance coinomat already has a service that allows you to instantly purchase and sell your crypto currency, and withdraw it directly to a credit card, directly from within the supernet client. Another team is building a decentralized poker application. Another team is building integration with Storj. A decentralized exchange system is also built in (with dynamic order books of any pair), with cool things like open source trade bots that people can code and fund and share in the profits. Another project someone is working on integrating small AI plugins directly into the blockchain. Basically, the possibilities here are endless. SuperNet is sort of like a cryptocurrency web browser.

Is it decentralised?  Or central-checkpointed PoS?

It is very decentralized. Given that it is not a "coin" but a collection of many coins and service. The software itself is a collection of nodes running a DHT network and extracting small fees for certain usages of the network. These fees are paid out to different actors in the system to provide incentives to keep the network running.

Does it include a floating alt-coin?  Does it incentivise developers to make example apps by paying them from a pre-mine?

BitcoinDark is one core coin of supernet however you are not required to hold them to use the system. The system is capable of just using them behind the scenes and is also a key ingredient to the teleport/telepathy systems which is an incredibly powerful method of anonymizing transactions and communications.  It also support communication with html files and may one day be more anonymous and private then Tor itself. White paper here: https://www.copy.com/s/x4mYj7Cy9tNtwwXg. This is currently being reviewed by Kristov Atlas. Developers are incentivised by owning assets in the differant supernet projects. Holders of these different assets receive dividends and rev share with all of the differant companies that join the supernet. This has the very interesting side effect of causing everyone to work together and help each other succeed, which is a beautiful sight in crypto. SuperNet is now number 16 on CMC. https://coinmarketcap.com/assets/supernet-unity/

Also, this isn't just pie in the sky, most of the code is complete and we expect a v1 release this month:
https://github.com/jl777/libjl777
https://github.com/jl777/btcd