1. Following Pokémon
2. 16 Gyms
3. Best Safari Zone
4. Pokéathlon
5. Gym Leader rematches
6. Elite Four + Champion rematches
7. Pokéwalker
8. GB Sounds
9. Voltorb Flip (actually logic-based, unlike slots, roulette etc.)
10. Battle Frontier (including, over Crystal:)
11. Battle Factory (Emerald, Platinum and HGSS's best feature in my opinion)
12. Battle Arcade
13. Battle Castle
14. Battle Hall
15. Increased levels from GSC
16. Keeps Crystal Suicune stuff
17. Expanded map (Cliff Edge, Seafoam Islands, Cerulean Cave, better Mt. Silver)
18. God-tier soundtrack (and soundfont)
19. Nice tilesets/palettes
20. Good events (Giovanni, Arceus)—they shouldn't have been events though
21. Fixed music over GSC (Upper Goldenrod Tunnel, Executives, Cinnabar Island etc.)
22. Latias/Latios roaming + Kyogre/Groudon/Rayquaza stuff
23. Miscellaneous things (Lance & Clair, Hoenn/Sinnoh Sound, Seals etc.)
24. Shiny Leaves
There's a lot of stuff in it.
Now of course, /vp/ absolutely despises them for the most part, thanks to the hipster brownie points it provides on 4chan for doing so.
When asked why, they'll say "muh level curve".
This is a meme complaint.
Levels work in relation to yours, so while Lance is only at level 50 the first time, your team will also be lower than it is in other games reaching the Champion.
They fixed the Kanto Gym Leader levels to be actually higher than Lance, what more do you want?
Do you mean "I want to instantly catch high-leveled wild Pokémon", or do you think XY were the series' hardest because Diantha's Gardevoir was level 68?