Not in any particular order
1) Build orders possibly most important to learn first (especially when really new to the game)
2) Keyboard shortcuts (N, D, H, Q, W, E, R, L, etc)
3) Use of waypoints and adequate scouting and anti-scouting
4) Dog control (started to play more RA2 now before sc2) and just getting your dog right to get that oil can win the game
5) Unit Control (with fodder or without? run past or stay put, aim at the weak tanks or let the tanks auto aim, move or stop, etc)
6) Effective splits and distractions (if you wanna eng, seal, ivan, etc).
7) Listening (including use of right mouse key for fast scroll)... I sometimes move my cursor to near the clients base and listen for sounds if for example I 'hear' his mcv moving I can act accordingly, including rockies, robots and similar. Especially on smaller maps where they may move like the alamo, HV (if they started in ore), urban rush, etc
8) Eco management. Simple maths. Using desos on yuri, killing miners, miner control early on, miner placement all pay big dividends in later game.
9) Traffic management - Occupying the right buildings, forcing tanks into single file (ala the film 300), getting the high positions on maps. If Yuri this is especially important to be behind trees, rocks, buildings, water so you can mag and land tanks on these.
10) Lame - Many players will use 'so-called' lame tactics like waypoint, engy, seals, etc but it helps to know how and when to use these.
11) Age and panic playing (thats how I like to call it) basically always looking around the field of play being able to key fast, scroll quickly, switch between views (units, defence tab, building, etc). Reaction time tends to peak mid teens to mid 20's.. So get some energy drinks and go mad
12) Team formations (especially when using planes if korea, allied) just to speed up your selection of units. Also T and P can come in handy for this as well as N and M.