-> SideChain does not require POW nor "blockchain" to work .. (but SC can use them, it is problem of SC-participants)
Same as altcoins
1) SC will only set-up some address/addresses in Bitcoin-world (lock bitcoins to prevent double-spent) -> and create SideChain-world
Same as altcoins (using an IPO/ICO/proof-of-burn)
2) In SC-world you can do SC-transactions (even by e-mail) -> build some hash-tree (you do not have to broadcast transactions to ALL WORLD and verify that ALL SEE THEM, b/c only participants matters)
Same as altcoins
3) SC-world has rules (Bitcoin users not affected) how to withdrawn/unlock bitcoins from scBTC (they must be able to create valid bitcoin transaction to unlock bitcoins).
This is slightly different from altcoins in that you can only exchange back to BTC if there is a willing buyer no an exchange, and the side chain concept provides something like an automated market maker that allows you to do so at some price and under some conditions (both of which can vary depending on the side-chain).
I see only subtle differences. Call it altcoins 2.0 if you like.