The following section contains information from a previous version of StarCraft II which is no longer valid.
The battlecruiser was deemed to be an essential unit from StarCraft and as such, it was deemed that it should return for its sequel. However, the battlecruiser was rarely seen in competitive StarCraft matches, while StarCraft II was to be designed specifically around skill-based e-sports. Dustin Browder and David Kim looked for ways to replace the battlecruiser because of this, but had no luck. The battlecruiser was considered so iconic that making the unit smaller or faster would change its identity.
The battlecruiser used to fire multiple laser bolts, each inflicting low damage. This attack could only target one unit, but once a target was dead, the rest of the blasts immediately moved to the next target, thus wasting no attack on any already-dead target.
The battlecruiser's abilities have undergone some changes in development, such as replacing its Plasma Torpedo ability with a Missile Barrage ability. This was changed as the plasma torpedoes did splash damage, thus overlapping with the role of the siege tank.
Plasma Torpedoes: An area-of-effect air-to-ground weapon which dealt damage over time and was effective against weak swarming units (like marines and zerglings). It added multiple small barrels to the front of the ship's hammerhead.
A battlecruiser will have the choice of being upgraded with either the Yamato Cannon, the Missile Barrage attack, or the Defensive Matrix, but cannot have more than one. Once one has been chosen, the battlecruiser may not switch. The upgrades are made individually (by selecting the battlecruiser). Upgrading the battlecruiser is very quick.
The weapon options other than the Yamato Cannon were removed from multiplayer by the beta. Blizzard found players rarely used the other options, and so they needlessly added complexity. However, they will be available in single-player. The Weapons Refit upgrade name was recycled for the Yamato Cannon upgrade.
Each version of the battlecruiser was treated as a different unit for UI purposes; double clicking on a Yamato-equipped battlecruiser would select all such battlecruisers but would not select battlecruisers with the Missile Barrage upgrade.
During development, the battlecruiser temporarily gained the Redline Reactor ability. This "doubles movement speed and increases acceleration of the Battlecruiser for 6 seconds". and was intended to enhance mobility in the late-game. The ability cost 100 energy and had a cooldown. The ability was removed later in development.
The battlecruiser was going to do extra damage against ground (+10 rather than +8), but it remained 8 damage to ground upon release.