For me the main requirements are:
* Backed up to cloud
* Has a mobile app that is easy to make small changes (for example if I'm doing an oil change on my car I need to be able to pull out my phone and log it in the wiki easily)
* Simple file format like markdown so that you're not locked into something proprietary (no need for crazy formatting options like MSWord)
* Option to export data out as a backup
* Allow uploading arbitrary files like PDFs, images, receipts etc...
* Nice to have: support for inline tables of data with simple calculations/sorting