I love the ship customization in Eclipse, each type of ship (scout, fighter, cruiser) has a certain number of squares on your board that you can put any ship competent into. You can add more engines so it can move further in one action, or computers so you're more likely to hit, or more guns so you roll more dice, or more shields so they are harder to destroy. You can find or research better components and all of your ships of the same class use the same blueprint.
I love how you can customize your units for how you want to play, but I'm not a fan of 4x games and wish there were other games with a similar mechanic.