I don't like the term "worst thing". It implies that there is something seriously wrong with core features. However I think there are things which detract from the enjoyment we can get out of the sim.
1.) AI
The AI has its moments. Both positively and negatively. My main gripe is with the bomber AI as it doesn't behave like a bomber AI but more like a bomber pilot who wanted to be a fighter jock. A B-17 doing loopings and barrel rolls? Huh?
Then there are the gunners. The only predictable thing about them is them being unpredictable - sniper now, blind and deaf the next second.
And finally there are the fighter AI issues which have already been brought up (shoulder-shooting, inability to use deflection etc).
2.) DM inconsistencies
Ever seen a Yak burn from its engine? Or a LaGG-3? The P-47 with a glass jaw? The "one hit and you're crawling slower than a snail" Fw 190? Nuff said.
3.) default textures and markings
There's so much diversity between the default skins (see Fw 190 A-6 vs F6F3, those are dimensions apart) that it takes away. I regularly use skins for all aircraft as I just can't stand the defaults anymore. They're partially 5 years old and that shows ... The markings are not the best, either, but that's also owed to the game's age.
And and and ... IMO there are a lot of tiny things which detract from the sim's enjoyment, but you probably won't see them unless you've spent a lot of time with the sim.
