my second journey through classic doom after realizing there's a map button meant I spent a whole lot less time wandering around and getting lost, which made that game way, way better, and I was already a fan of it!

I also had to try a few times to wrap my head around the first Final Fantasy's magic system (for some reason it just didnt click that magic level just goes up when you level), but now that I have I can summon earthquakes, and fights against large groups and enemies with specific weaknesses are no longer a headache like the first time I played it.

You might want to join this thread here:
https://www.gog.com/forum/general/games_you_were_100_prejudiced_against_and_ended_up_loving_to_bits

Warcraft 1 - Once I learned that there it actually had drag-select. You have to press and hold a button, and it's only 4 units, but still!

Fallout 2 - After I learned of the sniper perk. It's possibly overpowered, but it makes for a killer gunslinger playthrough. Previously I really struggled with the final oil rig, but once I replayed with the sniper perk it was almost too easy. Replayed Fallout 1 as well, also using sniper.

Haven't replayed Bio Menace lately, but since my last playthrough a couple of years ago I learned that Snake has actual super powers. That will completely change how I approach certain levels on my next playthrough of the series.

Oh?!!!! Ah! I found the game totally impossible and deleted and forgot about it but I just might try again now. Never thought it was a game where it would pay off to read the fine manual.

Final Fantasy Legend 3, when I realized that Beasts get a bonus when using martial arts; made that particular race so much more useful. Same can be said with early robot uses; certain Robot techniques do really good damage, allowing you to 1-hit kill most normal enemies in the first half of the game, and then later you get Dance..

Zelda: A Link to the Past: Once I started using the Exploration Glitch and other glitches, the game became so much more fun.

Exile/Avernum becomes a lot easier when you just bless and haste. Can't be hurt by things you've already smote first round.

Dying Light, didn't know how completing missions altered the world.

So on second play through too k more time to notice the changes.

Snake has super powers?

Final Fantasy 5:

Discovering that equipping certain abilities (like magic abilities or equip weapon abilities) will give you certain stats from the job that gives the abilities changed things significantly.

Also, every now and then, I discover more new things about the game, or realize that a certain unusual combo is actually useful.

In a manner of speaking yes:
https://youtu.be/ThnNSc9AjaU?t=650
I'll definitely use them next time, for example to get past those infernal slow moving slimes in the sewer level :P By the way, I don't find myself agreeing with the general sentiment towards the game in the video. Personally I think it's a great game :)

As much as I love it, Sid Meier's Colonization was not programmed with localization in mind. After years of playing the Spanish version of the game and being annoyed because two Native tribes (Aztecs and Iroquois) didn't show up in my favorite mode, the predesigned Americas map, I discovered it wasn't a feature, but a bug introduced by translations in other languages. The map uses the tribe name instead of IDs to place the Native settlements, so Natives whose names aren't exactly the same as their English ones just won't be generated. I think in the French version it was even worse.

This is very easy to fix by tweaking some text files, or even better, playing the English version, as this also avoids other bugs introduced by localization. Playing the Americas map with all the Natives makes a great change in immersion and gameplay. I can finally conquer Tenochtitlan!

Nowadays I try to play the original version of games (if I understand the language) whenever possible.

Legend of Dragoon

I missed Kongol dragoon stone on my first play and only get it at very last level making him a very weak character. On 2nd play through I got it early and he become one of my main attacker

I believe it was Cold Fear, didn't know you can react shoot enemies or what is called when they grab you. Last boss wasn't defeatable until you use that. So i beat the game and then replayed it using that skill.

Final Fantasy Mystic Quest: Discovering that Life, when cast on an enemy, would act as an instant death attack. (Note that this will not work on undead; note that this is different from the way the spell works in other FF games.)

If I recall/understand, mastered jobs grant this and more to Freelancers.