An irregularly updated list, roughly by level.

1. (Levels 1-?) The complete scarcity of ammunition for the guns I have.
2. A corollary: how much ammunition I have for guns I don’t have.
3. (Level 5) Supermutants, particularly those with large hammers.
4. (Level 6) The cost of Stimpacks.
5. (Level 7) Finding two dead power-armored Brotherhood members; being unable to pick up their power armor.
6. (Level 7) rounding corner and discovering inexplicably large Raider convention.
7. (Level 7) reloading save, discovering Supermutant Brutes with Chainguns have permanently occupied Anchorage Memorial high ground.
8. (Level 7) Mirelurks; esp. those inside the Anchorage Memorial.
9. (Level 8) whomsoever the frak the Talon Company is. Also: whomever hired them to stalk me.
10. (Level 9) Nothing is bull**** so far. Crazy!
11. (Level 10) Wait, these raider camps and super-mutant forts respawn? What is this, Far Cry 2? Argh.
12. (Level 11) Inventory Management. I need a mule to haul all this stuff around.
13. (Level 11) Speaking of mules, by this point in Fallout 1 or 2, I had a pack of allied NPCs. So far I haven’t even met anyone who might become an ally.
14. (Level 12) Trying to walk from Megaton -> Girdershade. The general level of danger in attempting to do the innocuous.
15. (Level 13) The National Archives.
16. (Level 13) Giant Radscorpions.
17. (Level 13) Lounges full of Super Mutant Masters with Miniguns. I’m just here for a good time, fellas. Please back it up a bit.
18. (Level 14) Whatever the holy hell is happening in Seward Square! It’s a post-apocalyptic armageddon near the Capitol Ruins.
19. (Level 15) Deathclaws. No kidding.
20. (Level 15) The Yao Guai tunnels. I don’t know what I was thinking.
21. (Level 16) Sentrybots.
22. (Level 17) Um, my own renaissance? I may have to discontinue this list, since I’m pwning fools left and right!
23. (Level 18) Escorting the free slaves from the Temple of the Union to their new home. Egads.
24. (Level 19+) Yep, it’s me. I’m totally the most broken thing in this gameworld, and with a rifle from 1862, to boot!