While the gist of SCII remains essentially the same, certain new features may throw an SCI player off. Here is a list of those features. Some of the advice may become obsolete as patches come out or new tactics are invented, and this article is designed to familiarize SCI players with SCII, not tell them how to play. I will begin with things that concern all kinds of players:

• Hotkeys have been changed to ones that are more reachable, for instance, the hotkey for Probe and Pylon is now <E>, not <P>. Therefore, if you are a random player like me, hotkeys <E>, <S>, <D> that you need to worry about at the beginning of the game are all positioned conveniently close to each other. Many hotkeys for old units have changed, so take note.
• Placing a rally point on a mineral will make all workers from the building actually mine the minerals. You no longer need to watch over your base every time a worker is made and manually tell them to mine.
• Workers distribute themselves among the minerals and don't gang up on one, so splitting them is not as important.
• There are commonly two Vespene Geysers at the base now. This means you need to build and harvest both to get approximately the same amount as in SCI, thus spending slightly more minerals and workers. The cost for the Extractor, Assimilator, and Refinery, therefore, has been lowered.
• The new Xel'Naga towers, ignored by many players, permit you to see a very large amount of the map often at as little as 150 minerals for 3 Marines, or even 75 if you are Zerg. All you need to do to activate a Xel'Naga tower is place a unit near it. While these towers will probably not tell you exactly what the player is doing, you may still see the forces he is sending, and from which direction, especially important on multi-pathed maps like Desert Oasis.
• You may now hotkey multiple buildings of the same type with one hotkey, and creating units within that selection will make them evenly between buildings. I highly recommend grouping worker buildings on the same hotkey when you expand.
• The unit selection limit now seems to be infinite, therefore, you won't need multiple hotkeys to just hold 2 groups of 12. You still want to do so for Micropurposes, of course (ground and air, for instance). You can also press <Tab> to select a different caster group in your selection, and casting something with a group of casters will only make one of them cast, so you can now safely cast with 7 High Templars without worrying they'll all waste their energy.
• Pressing <Shift> when selecting an ability will keep it selected so that you can cast the ability multiple times without pressing the hotkey over and over again. This also works for Protoss Warp Gates.
• You don't need to manually tell workers who exactly to attack anymore. Workers will now attack enemy units if you simply attack move a location where enemy units are situated. This makes them considerably more effective against early game rushes, especially Zerglings.
• New kinds of armor have been added, particularly Light and Armored, which are very important and should be taken in consideration when you're deciding which units to build or what to counter the enemy army with.
• The expansion priorities have changed somewhat. Zerg seem to be quite able to survive with one base, and it may be a good idea for Terran and Protoss to expand if Zerg expands. In other words, do not rely as heavily on SCI when considering expanding.

Now onto race specific tips:

Terran:

• The build order I use is 9 Supply Depot, 12 Barracks.
• The new Reactor/Tech Lab addon mechanic is both cool and annoying at the same time. For one, you probably want to add something to the Barracks you use to block your ramp, which often makes Blocking the ramp extremely awkward, depending on the map. Furthermore, you cannot properly make units in two buildings of the same type if they have different addons, one will be selected as priority and you need to <Tab> the other. On the bright side, all buildings can have the same 2 addons, which means you can generate some interesting tactics with building an addon on a Barracks, lifting it, and putting a Factory there, enabling you to, for instance, get Siege Tanks faster.
• The Command Center can be upgraded to an Orbital Command or a Planetary Fortress. The initial Command Center will almost always become an Orbital Command, since it has a resource gathering mechanic, Call Down MULE, an efficient worker. It also allows Scanning, and it can give you extra supply if you forgot to build a Depot. All the energy is shared, though, so Scanning something will be equivalent to losing minerals. A Planetary Fortress can actually attack enemy ground units, but cannot lift off. Similar to addons, grouped Command Centers of different types will need to be <Tab>'ed if you intend to produce SCV's at them.
• As a race with the most different producer buildings and various modifications, Terran is the most crazy in terms of Macro.
• Terran can get Nukes considerably earlier since Ghosts are now lower tier.
• You can no longer use an Engineering Bay for scouting because it can't lift off. It now also has building upgrades, for those of you who like turtling.

Terran Addons

Zerg:

• The build order I generally use and consider pretty normal is 10 Overlord, 13 Pool.
• Overlords are no longer Detectors. To become Detectors, they need to be morphed to an Overseer, available after Lair.
• A Hatchery now has two rally points, one for workers, one for everyone else. Setting a rally point on minerals will define the worker rally point, and setting a rally point on the ground will define the normal one.
• Zerg now has a new Queen mechanic, which is crucial. While normally the answer to too many minerals and too little Larvae was more Hatcheries, in SCII it may very well be more Queens. A Queen possesses a spell that permits it to generate more Larvae at the Hatchery. They are so important that I suggest hotkeying them separately so that you can quickly tell them to Spawn Larvae at the Hatchery they're close to. The Hatchery should have the Larva Spawn effect on it continually. The standard is 1 Queen per Hatchery, but more can be made, and a Queen costs half the cost of the Hatchery while also providing other bonuses, including attacking enemy units or being able to heal your own units and buildings.
• Queen is the lowest tier Zerg unit that can attack air units. While not much, it's sufficient to kill an enemy Overlord.
• Vice versa, if you're fighting a Zerg player, they are no longer as defenseless in early game, and keeping an Overlord there too long may cost you.
• Spore Crawlers and Spine Crawlers are the new Colonies, and can now move around the creep to root into a new position.
• Starting with Lair, Zerg can use Overlords to spew creep anywhere on the map. This means that you do not need to build a Hatchery to generate creep somewhere and build something there. Yes, you can stick a Spire in a corner of the map and nobody needs to know. The Queen can also make Creep Tumors on existing Creep to expand it.
• Zerg had some unit and tier changes, and one thing to note is that Zerg lost early game units that attack air (excluding Spore Crawlers). Besides the Queen, Zerg needs a Lair to get anything that can attack air, including Hydralisks, who have been moved up.
• Zerg has little to worry about in terms of Macro besides groups of Hatcheries and Queens.

Zerg Have Two Rally Points

Protoss:

• Build order similar to Terran, 8 Pylon, 12 Gateway.
• Protoss has Chrono Boost, available at the nexus, that makes any building work faster. This can let you make Probes faster early game, decrease Cooldown on Warp Gates, or simply speed up an upgrade.
• The Warp Gate mechanic, researched at Cybernetics Core, will let you make units out of Gateways faster and exactly where you want them. Keep in mind, though, that warped in units are vulnerable until they're done. Any Gateways that were converted to a Warp Gate will be added to a special hotkey group <W>. This means there is no need to hotkey Gateways after you have gotten the upgrade, assuming you remember to press <G> to turn the Gateway into a Warp Gate.
• Warp Gate mechanic means you can warp units anywhere where there is a Pylon (or a Warp Prism). Beware of enemy Pylons and build your own near the enemy base to make them nervous.
• Warp Prisms can both function as Pylons, or as Transport units. Units from the Robotics Bay cannot be warped in and are usually the ones transported.