I find the biggest source of lock-in is that it's really hard to move a bunch of data. The longer you stay, the harder it is to leave, because the impact (downtime, slowness, resources allocated to moving data instead of building your business) on your product/service is likely to be bigger.

As you research this, don't neglect the cost of attempting to remain vendor agnostic. Every level of abstraction adds new costs (for some people it's worth it, no doubt!). Sometimes it's more efficient to just go all-in with a vendor.