I've been running D&D since a year or two since 2nd ed came out.
I've been running D&D 3.X since before the Monster Manual came out.
The XP system of the 3.X system is just so much easier for me to work with.
The wonky XP tracks were just another number to keep track of.
Each class rose a different rate (Rogues shoot up to begin with) as a balancing mechanic.
THAT was one of the worse things about 2nd ed.
They used varying levels and abilites to try to create balance. Sure an Elf gets all this cool stuff, but at level 15 he stops & the human starts to be cool.
In other words...
For 75% of the games I played in the balancing factor would never come into play as we rarely ran high enough for it to matter. And, if we did, the demi-humans would get bored (as they got nothing) and we'd start over.
From a DM standpoint, the amount of time I spend figuring out challenges & adventures has dropped about 75%. I can concentrate more on the Adventure & less on the Mechanics of the challenges.
As for your scenario: (Everyone learns somthing from the encounter in a different way)
1. They ALL ALWAYS get the same level of learning from an encounter. The 20th level Fighter gets the same XP from slaying the Ogre the 1st level Mage gets, that the 3rd level rogue gets. There is no scaling based on previous experience. In 3rd, higher level characters not only need more & more XP to go up in level, facing threate beneath them offers a lower & lower reward.
Why should Uthgat the 20th level Fighter get 15 XP for killing his 3,000 Goblin as he got when he killed his first?
2. The (optional) individual XP awards sucked. Sucked much.
A. Fighters got Hit Dice Killed. They cleaned up.
B. Rogues got GP FOR TRESURE THEY KEPT FROM THE REST OF THE PARTY (read the fine print). Rogues got rewarded for stiffing it to the rest of the party. (I killed more than a few XP greedy party members).
C. Clerics & WIzards got Spell XP. As clerics did all the Healing, they got quite a bit of XP. As mages had a pretty limited spell access (esp at lower levels) they tended to get hosed.
I've ran 8-9 different systems over 2 decades. I've played in dozens more. I can say there are XP systems I like better then 3rd ed (I prefer Champions & White Wolf System).
I can say that I can't think of a System I detested more than the old D&D system.