It seems to me that the diplomatically challenged races in all Master of Orion games seem to be the ones that take over the galaxy due to their other advantages. I advise all beginner players to watch out for these races.
One race of particular note is the Harvesters. Beginners should always watch out for them. Contrary to what I said in the system taking strategy, it should be the primary goal of all players to remove the harvesters as quickly as possible as later in the game they can be a large thorn in their side. The Harvesters are the most scary race later in the game as they usually work up large empires full of powerful ships but early in the game they are as vulnerable as anyone.
This strategy also works for the Silicoids in MOO2. Since they are also 'diplomatically challenged' you should aim to remove them as early as possible.