Just as an example, which would people prefer: “Outpost” or “The Stateful Inspection Firewall made by 67.15.103.130”?
That's just apples and oranges, though. Outpost doesn't use the Kerio engine. Norton doesn't use the Sygate engine. They each have their own entirley different engine.

IPtables however is it's own engine, and doesn't need a frontend to function. Which is where my point is. You are just listing front-end interfaces for IPTables versus different firewall engines entirley. IPTables and pf, for example, are two different firewall engines. Just like Kerio and Outpost are two different firewall engines.

Firestarter, for example, is nothing more than a gui interface for the IPTables engine (which can be ran witout firestarter, because all firestarter is doing (like 99% if the iptable front ends) is running the iptable command-line for you behind the scenes.

Engine versus front-end. You can't compare Windows' products to Linux one's in this regard.