Myself, Thomas V (Electrum) and Marek (Trezor) got together to make sure our BIP32 wallet structures would be compatible - and I discovered that only I was planning to use the default structure.

Because I'm hopeful that we can get a lot of interoperability between wallets with regards to importing 12-words paper wallets, we brainstormed to find a structure acceptable to everyone and ended up with:

/m/cointype/reserved'/account'/change/n

The extra levels require some explanation:

cointype: This is zero for Bitcoin. This is here to support two things, one is supporting alt coins based off the same root seed. Right now nobody seemed very bothered about alt coins but sometimes feature requests do come in for this. Arguably there is no need and alt coins could just use the same keys as Bitcoin, but it may help avoid confusion if they don't.

More usefully, cointype can distinguish between keys intended for things like multisig outputs, e.g. for watchdog services. This means if your wallet does not know about the extra protocol layers involved in this, it can still import the "raw" money and it will just ignore/not see the keys used in more complex transactions.
reserved is for "other stuff". I actually don't recall why we ended up with this. It may have been intended to split out multisig outputs etc from cointype. Marek, Thomas?
account is for keeping essentially wallets-within-a-wallet to avoid mixing of coins. If you want that.
change is 0 for receiving addresses, 1 for change addresses.
n is the actual key index

For bitcoinj we're targeting a deliberately limited feature set for hdw v1 so I would just set the first three values all to zero and that is a perfectly fine way to be compatible.

The goal here is that the same seed can be written down once, and meet all the users needs, whilst still allowing some drift between what wallets support.

Pieter made the I think valid point that you can't really encode how keys are meant to be used into just an HDW hierarchy and normally you'd need some metadata as well. However, I feel interop between wallets is more important than arriving at the most perfect possible arrangement, which feels a little like bikeshedding, so I'm happy to just go with the flow on this one.