One thing people will have to watch for, as they spend their spin-off coins, is privacy implications. If you have a bitcoin address A, replicated to various spin-offs, then each time you notice you have a non-zero balance in some spin-off and decide to spend it, new spendings from address A go out into the public domain. (It might not be called "A" in the spin-off's preferred widely-promulgated syntax - which might use a different prefix, error-correcting suffix, etc. than bitcoin's - but the underlying pubkeyhash must, of necessity, be the same.) The effect is the same as if someone working entirely within one coin accumulated multiple txouts to one address and then spent them to different recipients on different occasions. Blockchain-watchers can learn a lot from this, which is why it's not recommended. Well, with spin-offs, they can learn the same sort of things.

Naturally, coinjoin and the like will spring up in the various spin-off communities, and people who know what they're doing can mitigate the privacy dangers. There is, though, the worry that any particular spin-off might have a rather small community, and not really support good coinjoin mixing.

Having said all that, I must say I'm really impressed with the whole spin-off concept. So much so, that after dithering for a while, I've decided to abandon my "Splash: Ripple without the pre-mine" project, and leave it to someone else to fork Ripple in spin-off style, i.e. starting with the Bitcoin UTXO-set - or, to be precise, the subset that translates into the rather impoverished Ripple address syntax (no multi-sig, no scripts, just single pubkeyhashes).

So, grand announcement time: I hereby relinquish the name "Splash", the slogan "Ripple without the pre-mine", and the currency abbreviation "XSP" to whoever is first to do this. (Or they could do it in Counterparty's style, with XSP minted by BTC being burned. Basically I relinquish to whoever is first to do a Ripple fork without an allocation by software special-case fiat to the developers, or to any other special cases.)

So there you are! You can tell I'm really taken with the spin-off idea, can't you? But please, everybody, bear those privacy aspects in mind!