When Blizzard designed StarCraft 2, one of their objectives was to make it good as an e-sport, so it was important that the presentation communicates information effectively, for players, commentators and spectators. Some principles StarCraft 2 seems to follow are:
1) Favor putting stuff on the map rather than in menus
2) Unit upgrades should be discernible just by looking at the unit
3) Everything a unit or building does should be visible. Powerful or weird abilities should be eye-catching.
Let's look at some examples of these rules.
1) All resources are actual objects on the map: minerals and gas. Minerals running low become sparser in appearance. Active geysers are green and lively, depleted geysers are red and static. Unit supply is represented as buildings (supply depots, pylons) or units (overlords). Radar range (in case of Terran scan or sensor tower) is very obvious on the map. Zerg's concept of a zone of control is a living substance that can advance with small buildings (creep tumors). Pylon coverage overlay is made visible by simply selecting any pylon.
2) Although StarCraft 2 has a few abstract upgrades for each race (+1 melee/range/air attacks, +1 ground/air armor, etc), most upgrades provide tangible, observable tactical advantages.
- Unit speed is used often: zerglings, banelings, hydralisks, reapers (cut in HotS though), zealot charge, warp prisms, etc. This is often a huge, vital upgrade. Upgraded zerglings develop wings and their animation is adjusted to reflect this; banelings gain the ability to roll around.
Other examples: range upgrade (Hydralisks, Colossus), stealth (Banshee), movement while burrowed (Roaches). Marines' combat shield equips the units with visible combat shields (effect is increased health). Hellion damage upgrades turns its flame from red to blue (it's hotter!). The Marauder's concussive shells dramatically slows its target down.
StarCraft 2 tends to favor active over passive abilities due to its focus on micro: short-range teleportation, burrowing, infantry stim (temp boost at the cost of health). While this is not a design goal of AotS as I understand it, autocast abilities on cooldowns could make sense. SC2 example: Zealot charge.
3) Lots of things from SC1 were made more visually obvious in SC2. The Sunken Colony's underground spike was replaced with the Spine Crawler's long whip. Lurkers were removed (although set to make a comeback in the next expansion). The Guardian's simple green ball animation was replaced with the Broodlord's broodlings, shooting actual units (how cool is that?). The medic infantry unit was replaced with airships casting green healing rays from above. The Dark Archon's mind control was replaced by the Infestor's Neural Parasite, which draws a tentacle from the infestor to its target for the duration of the spell. In Heart of the Swarm, an animation was added to creep tumor spawning to make it obvious which creep tumor was spawning a new one. SC2 tries its best to make cause and effect very obvious.
Tech choices are obvious the moment they are made in SC2. Zerg researching Lair has their main hatchery pulsating violently until the upgrade is done, and the Lair looks very different from the Hatchery. Terran's choice of attachements to their initial barracks tells a lot about their upcoming tech choices (tech lab? perhaps banshees are coming). Choosing a tech path usually means building something specific that can be scouted: a robotics bay, a roach warren, an engineering bay, etc. This enables players to try and hide their tech by placing buildings in unexpected locations on the map, creating interesting strategy and making scouting more useful. Many games were lost or won because of a hidden Dark Shrine or Stargate.
I could spend more time and find more examples, but I hope I've made the point that StarCraft 2 has a very refined, well-designed visual presentation, and this greatly enhances its gameplay, makes it fun to watch and easy to learn - while remaining of course hard as hell to master. I hope some of these ideas make their way into AotS, which currently favors hiding upgrades and resources in menus.
On a side-note, Age of Empires 2 has a similar thing where advancing in ages changes the appearance of every building. Major unit upgrades (Crossbowmen, Paladins, etc) change the appearance of the units completely too. Chemistry makes every missile be on fire - a bit absurd, but fun. Forests actually recede as you exploit them for wood; fish disappears from the rivers.