The "everything is a string" protects the protocol from the variable implementations of integers on different platforms. A string can be safely read by 8 bit, 16 bit, 32 bit and 64 bit platforms.

The perspective is quite intentionally BTC-centric. If someone wants to make a LTC-centric exchange they can either adapt the protocol or make a new one. This is BTC-UFX, not ???-UFX.

There are no return values for BUY/SELL etc because the execution side and the order relay side are independent. At the time the exchange accepts an order it does not yet know if/how it will be executed. This is as it should be.

The exchanges are free to use their own time. As long as they maintain the same timezones in their own STATJSON responses all is well.

Ideally exchanges would switch to at least asset-denomination that is compatible, for sure.

Before trying to create new standard, one should consider existing ones and make clear that they are inaccurate and why. You have mentioned FIX protocol yourself, why not use it instead?

Technical merits should be reasonably justified, for example you decided to use only a "everything is a string" subset of JSON and with embedded checksums. This adds additional effort necessary to emit/parse JSON data -> risk of bugs/incompatibilities that need to be outweighed by benefits, and more possible solutions considered.

There is no extension mechanism outlined, for example exchange may want to provide ability to buy/sell the same asset in litecoins.

Specification of BUY,SELL,CANCEL,EXERCISE,etc. return value is missing. I can from practice say it is very important for client to be able to reliably check how much items were actually accepted for the order, whether cancel was accepted and what was the exact result of exercise.

Above 2 points could easily be solved by adopting JSONRPC or similar existing protocol.

All dates should be specified as UTC or formatted with timezone.

And, on broader note, this interface should be adopted by multiple exchanges, much wanted function will be possiblity to move assets between them. Maybe it would be even beneficial in long term for MPEx to fund research in this direction.