I divide IOTs into five categories:
Mini-IOTs: very simple and very straightforward, often little more than a map. Players freely create their own fantasy countries and generally do whatever.
Basic: a map and a few rules, mostly regarding combat. The original IOT by Taniciusfox is an example of this.
Detailed: a bit more complex, with the addition of rules governing diplomacy, income, trade, espionage, events and special projects. These often have extensive backstory, include NPCs and a more complicated combat system. Examples include Sonereal's Revolution IOT, Thorvald's IOTIV, TK's IOTVI.
Complex: yet more complicated, comparable to some of the simpler NESes or lazyNESes. Examples include Imperium Universalis, Sons of Mars, and the ill-fated "community" IOTV.
Super-IOTs: these are the largest and most complex IOTs. Iron and Blood is in this category. Look for large number of players, a very long game thread, a detailed game world and extensive roleplaying, multiple very large social groups, and perhaps an off-site wiki dedicated to the game.