First you should always run backups of your *.wallet files. So that if your HDD crash you can recover your bitcoins and the private keys to be able to spend them.
In regard to be able to switch wallet app if the development stops, I feel that support for the Wallet import format(WIF) is the only requirement you need to care about. It look like this:
5KJvsngHeMpm884wtkJNzQGaCErckhHJBGFsvd3VyK5qMZXj3hS (5 in front)
or the compressed
L3p8oAcQTtuokSCRHQ7i4MhjWc9zornvpJLfmg62sYpLRJF9woSu (L or K in front)
In fact every app can export to this format, regardless of any special feature, like Deterministic Wallets. There are also hybrid web wallets that export to this format.
But there are also web wallets where you are not truly owner of your bitcoin address, because they do not allow export with this format. In fact, there is no way of being sure that the company don't have access to your private keys.
can you list some of the apps and online wallets that support this universal export format? – Patoshi パトシ – 2013-10-16T17:04:14.153
Sure, I will post it in a new Answer here... it will take some time to research this :-) – Jonas Oestman – 2013-10-16T19:08:50.460