First, a clarification: Homeworld 2 is NOT Homeworld with better graphics and another story line; it is a completely new game. It has many similar concepts and basic designs from Homeworld, and it integrates the same history, but it is 100 years in the future (reletive to Homeworld) so don't expect it to be the same.

I'll start with the firing-back-at-insults section. Many people claim HW2 sucks, so I'll run you over the truth of things:

Number one insult: "In Homeworld I can control my fighters, in Homeworld 2 I can't"
This is BS. In Homeworld 2 you have a squadron system, grouping your fighters into different size squadrons, depending on the type of fighter. These squads generally range from 9 to 3 fighters in size, the more powerful fighters and corvettes having lower group sizes (but all a similar cost, around 400-700 RU). These squads fight together in formation in battle, providing much more effective firepower and organization.
Now some facts: When will you need a single fighter, ever? What good is a single interceptor or bomber? organizing into units means that I never lose one of my fighters somewhere in a battle. This can all add up to tip the balance on a very small map, especially if all the fighters break up and scatter across the map. So, organizing into squadrons allows you to keep track of them. They do often scatter when opposing area-effect weaponry (like flak turrets) but then they regroup.

Number 2 insult: "Homeworld 2 has no strategy!"
If you say this you deserve to be SHOT. HW2 has many more features which involve strategy, including shipyards, subsystems, more dynamic maps, better and more complex environments... you get the idea. In Homeworld, you often have to worry "how can I move my ships around so they dont get picked off" in Homeworld 2 you worry about "Here are my options, how will they effect the future of the game, will it work? What are the pros and cons?" It's more strategy in Homeworld 2. You can use much more tactics. In Homeworld, you use several basic strategies which I can name right now: swarming, capital ship rush, or a mush of both. Battle tactics: not very sophisticated, just click on a ship and boom its dead.
Homeworld 2: you can surgically strike a ship in battle. Lets say you have barely enough resources to win. You get a few bombers, and go and take out the enemy carrier's resource drop-off point. Kill the collectors. Then get a Marine Frigate in there to capture the carrier. By the time you've captured it, the resource drop-off is repaired, and you probably have enough defense there to ward off attackers. Then you get your own collectors in there to collect using the enemies pocket; there you go. In Homeworld, you just choose which pocket is yours and collect there all game, no attacks on it, its pure fleet-to-fleet. In Homeworld 2, you can get it ALL. You also have an improved diplomacy system, like transferring ships

now modding: modding and mapping for Homeworld is INCREDIBLY easy. Mabye not on the same scale as other games, but its pretty damned easy to do. mapping is REALLY good with its LuaC capabilities, programming a map instead of telling it (in 3D coordinate system) where to place each little pebble.

id go on but i gotta sleep.

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