The illumination rules bug me - bright light so many squares then DARKNESS. How does that work with low light vision? The rules don't tell me... I know what they are trying to do making the game faster and all, but telling my eladrin and elf players that they get no extra benefit to see beyound the illuminated torch area because the bright light "screws up their IR vision cameras" was painful.
The concealment vs line of sight stuff seems like it was all changed last minute so there are lots of little inconsistencies sticking out all over the place.
I hate how there are no odd powers for the non ranger classes to take to let them do 2[W] damage with ranged weapons. I know the ranger is the "ranged guy", but honestly every class should have a couple of options so those who want to very occasionally shoot a ranged weapon don't feel so gimped. Basic attacks vs powers beyond low levels just doesn't feel like a fair comparison.
It bugs me how the very best Rituals do not require pc's to be trained in the requisite skill. Raise dead, enchant magic item etc. don't rely on the d20 skill checks at all, which makes me sad because something interesting could have been done there. But then I am a sadistic DM who loved rolling up the reincarnation table in 3.5e, so maybe they did the right thing there.
I love most of the 4e sysem and how fun, fast and dynamic things can be, but there are so many crappy magic items with crappy conditional daily powers that just add mental overhead to the system for so little payoff. Some of them feel like a really cool thing to do once a day that makes for memorable moments like the level 7 cold melee gloves from adventurers vault or jumping off a cliff with catstep boots, but there are so many useless arms slot items that it feels like they haven't figured out what they are doing with that slot yet.
I just wish rings were the one slot where the gloves came off and they just allowed them to do super cool things again, several times a day. So many rings suck so bad, yet they were originally going to be "paragon only" items so cool players couldn't wait to get them! What went wrong?
Finally, insisting that both sides are aware of each side before a combat unless one side is using stealth just bugs me for overland encounters. The section is written for dungeons really, I just wish there was a little more substance there to provide more variety to beginning encounters at different distances or with one side not quite aware of the other due to lower perception scores. I don't want my PC's to be going everywhere in super slow stealth mode if they want to suprise the monsters in a random encounter.
Basically the nail of all of this is - sometimes they tried making things so simple, they wound up making things more difficult to play, rather than less.