The most important thing about ERP systems is - customization. IE: Scripting (netsuite), or even core programming (Sage100 for example). Not just user defined fields but workflows and being able to override and hook into core parts of the system. Say you want to override your cost basis for a certain productline or maybe serve different prices based on the shipping warehouse.
How do you approach that?
EDIT:
How do you handle the finances, G/L auditing, and allt hat financial wizardy?
customers.. they love PDFs. How do you handle generating things like pick lists, invoice sheets, etc? templating?
EDIT2:
One thing big ERPs provide is basically a cohesive way to extend. Not just an API in a RESTFUL sense, it's more akin to an IDE or like GTK+, there's almost everything rolled in for extension inside the 'world view' of the ERP. Every ERP makes some decisions and the rest of the world may flow, be that how you issue credit memos, handle multiple financial entities (do they all have the same chart of accounts? do they all have the same modules? Are there shared users) and so forth. How do you approach that? IE: when you need to slap something like PO's on top of AP, so basically AP + Items and a whole bunch more, does that "flow"? Do you receive those goods and they end up in your inventory for AR? etc.... Having an extensible system is great for addons and consulting, but the bigger piece is - for you, letting your ERP grow and add capabilities.
EDIT3: "Stripe" is not a billing techstack. Mostly CFOs make decisions about billing options, not us lowly tech monkies. Do you support pluggable vaults or anything akin?