I have the paint can on Bold - Blue whenever possible; if the server's colorset doesn't have a similar color in it, I instantly hate that colorset.
I always make sure to clear my ghost brick when I'm done. This gets annoying sometimes because I might clear it when I'm not finished yet.
In the bricks bar, I keep it scrolled to the first slot whenever I'm not building.
In the bricks menu, when I'm done building I always make sure I scroll back up to the top for each tab.
I always scroll to the wrench before closing the tool hud.
I absolutely cannot stand having fall damage and jets at the same time.
I occasionally check the player list, sort it by BL IDs, and see who has the lowest ID just out of curiosity.
If someone has an auto-greeter, I always get the urge to say "connected." in chat.
Lastly, I tend to press esc to close the brick menu. Quite annoying as I always have to re-open it and click "done" because of this habit.