Much of it has been forgotten, but some of it still dooes come to mind (and I started around 2016-17)…
I didn’t have this on my own PC, so I just used the public account at a video game lounge. Figured I go Swann and Karax since those were the furthest down that were unlocked, so I’d get an extra “bump” there (assuming the COs furthest down the line were better, more $$, and “more fancy”)
Later on when I reverted to my own Sc2 account (vg lounge couldn’t authorize any IAP when Nova and Stukov were on sale… $2.50 apiece), I learned that the Top Bar/Panel could be set as keyboard shortcuts (before they had default shortcut keys)!! This made playing Karax MUCH EASIER! I also ended up going through full mastery (lv1 to lv90), twice!
Unlocking the blue talents was exciting! I get it when people ask about fast tracking XP and leveling up b/c I was eager to try out new talents (esp. Karax lv11/Kahlai Ingeniuty).
Smacking yourself for a game that’s starting, but forgetting to assign your newly earned Mastery point 
.
–Zagara was my 3rd CO I played, after Swann and Karax. I was just shocked how fast she fully saturates (within 2 minutes)! I didn’t think I’d like using her since her “swarm style” didn’t seem attractive.
–Abby’s UE were so strong, I honestly thought that was a mistake, and it was only a matter of time before Blizz would nerf them.
–Part & Parcel… I failed this the first time b/c despite Davis telling you what to do, I was still confused 
–First game of Nova on Part & Parcel… I spent the entire game reading through all of her command card buttons and mouse over text. Failed the mission because I didn’t go out and hunt for parts! Gotta say, Nova is well designed.
–Fenix… loved having the bug where I could infinite cloak on Arbiter suit b/c it didn’t deduct any energy! First time leveling him up, also never learned how Avenging Protocol nor Tactical Data Web worked. (I’m not even sure I made that many Champions TBH :\ )
–Mengsk was the only CO I both purchased and played on their opening weekend. Crazy waiting 3 to 7+ minutes trying to get into a game (since everyone else wanted to try him out as well, but the game can’t start a game with the same CO). Some folks were clever or otherwise, stumbled upon queueing up for Brutal games as lv1 to get into a game much MUCH quicker.
–Was excited to be around for the release of 2 maps (P&P, and CoD)
). When I went to play it at a vg lounge, I was just going to do some Arcade/Custom games (coop preferably), and some Vs. AI games (especially since I was new to LotV at that point). That’s when I noticed there was a tab for Coop.
marines, 
firebats, medics, except the marauder wasn’t in it, good stuff blowing up things using siege tanks and vulture spider 
mines 
, battlecruisers, won my first Brutal game as level 7 Jim Raynor with a level 9 Vorazun ally who helped tremendously with the blackhole 
as I spammed MULES, vikings and vultures/ spider mines on Oblivion Express before I knew you can rapid-fire. I think I broke the C key on the old keyboard. I ended up getting a new keyboard after getting things mixed up using a different key since it broke. To this day the vulture spider mines live up to my dearest sc2 coop memories as Jim Raynor. I cheese most maps running around with packs of vultures and vikings and spam Cs after Cs until it is a cease-fire.
. I love earning extra resources for me and my ally, loved mass hydras and seldom 5-7 ultras packed into 6 omega worms I could spam for days and as living fleshy detector walls on dead of night 
and lurkers, Mmmm love them! Her zerglings+muta+brood lords were epic too. I was a super Kerrigan fan until I purchased abathur then dehaka. I still play Kerrigan decently though, if the random button blesses/curses me with it.
templars. Rarely ever used the meme-sake Reavers. Only, out of pure whim, surrendered massive amount of resources to build a robotic 
worm-brigade on Miner Evac splitting them up so they don’t overkill a dead soul. Oh, I hated the Miner Evac side missions, especially the bottom right one on Brutal missions. I would just save up a dozen hundred minerals and send zealots to them and use the solar bombardment on them. When I didn’t have them? I would lose half, if not most of my army trying to get that measly side obj exp, if my ally wouldn’t offer up some live sacrificial pawns 
for the death ritual ahead.
I get for boosting up our vespene income up a notch or butter-scotch 
- totally worth giving up some couple hundred minerals. Only those who think it’s free are the ones that I couldn’t stand. Heck, early game, some guys see their teammate is Swann, they feel that they are entitled to an automatic gas boost as soon as they finish building their geyser constructs as if it was the Swann player’s one-and-only function in life was to know exactly when and down to the final second when their gas buildings are available to drop gas harvester drones on. Some would get 
angry 
, in turn, making me angry 
of suicidal bombings/bombers so I skipped playing her. Perchance, the random commander button had chosen Zagara for me was whence I had realized she wasn’t bad. She wasn’t some lunatic commander who sends her army to death. She IS what she IS. She was born to command the army of volatile things! I loved weakening down big boss unit or trains using the corruptor ability. Oh lordie lord, before the range buff from one of the 2019 patches or somewhere there can’t remember when, I used to meme-purpose the bile launcher on centre cliff and top expo cliff of OE and as defensive DoN strat. Her Masteries have been bugged for the longest time. 
kaboomza damage! Ohhhhhhh hnnnnnnnnnnnnnnnng~ Lv. 10 TIMESTOPpppppppp! Stahp impressing meeeee we you masta-racer I’d hafta paste mustard-hey sirrr *purr (lol whut). Good times. But needs micro on higher difficulty to avoid splash deaths.
) before that. I wouldn’t complain about the Energizer Reclaimation ability, however. I would mass Zealots and Energizers and prefer to (over Terran mechs) control enemy Toss mechs like there was no tomorrow.
monster 
. The happy ending. Got me so enticed over Kerrigan. The Ultimate Evolutions were bomb! But I liked spamming Corrosive Biles and vipers more! Recently realized you can manually prevent a certain unit from turning into an UE by right clicking the -born/spawned-that-way unit who has turning into UE autocast at birth. Oh my ill-begotten losses!