As mentioned, BUGS! Especially when you're "in the zone" and some stupid twathead of a bug presents itself. Then your creativity is drained away every half hour spent trying to find it. Then you find it's the most idiotic thing ever. Like you left in some debug code that changes a variable every tick yet you're left wondering wtf that variable that you're setting elsewhere isn't working the way it should! Thank god for debug mode though or I'd have spent 5 times as long finding them..
Menus yeah, they can be boring to implement, but depends what you're doing with them. You can make them like any other part of the game and animate them, flashy effects and so on. They're probably boring to do because you're making them boring ;p You're being creative with the rest of the game... why not with the menus too?
I think for me though, it's feature creep. No matter how hard I resist or do small docs with my ideas.. it always seems to make my dev time much longer. Even if I've set in stone what the game is going to have and what it's not, I'll always think of new ideas that I just HAVE to put in.. ;p
EDIT> How could I forget this..... I f*cking HATE all the different device hardware, screen sizes / resolutions.. then there's browser quirks... If I had one wish, I'd wish for 3 more, and use one of those to wish that every damn device and browser worked the same way!