The difference, if anyone's still playing along, is what action the device takes. An IDS (detection system) is a monitoring and alerting device; traffic still gets through. An IPS (prevention system) drops the flagged traffic.
As you alluded to, the distinction between IDS and IPS is largely configuration and mode of operation.
Years ago, IDS and IPS were separate products, where IDS was the earlier, more primitive version of the other. Now-a-days, you are buying an IPS, which is run either in alerting mode (operating like an IDS) or in "shunning" mode, where the device tracks some defensive action (such as dropping traffic, bandwidth throttling, blacking the IP for a fixed period of time, etc).
"Shunning" mode can be dangerous, since you are essentially building in a feature to "Deny service to X for Y amount of time" into your network.
Attackers can spoof attacks to deliberately trigger the shunning of legitimate users. Because of this, it is less common to see an IDS/IPS with shunning enabled in production. It depends on where "Access to service for legitimate users" and "stopping and possibly hurting attackers" fall on the priorities list.
