Regex. I have been aware of its existence for probably a year or so, and can generally recognize when it should be utilized, but I am far from being able to make the leap from a string-pattern-identifying algorithm in my brain straight to a regex. I usually have to - at that point - refer to a book or online resource.
Front-end design. I can make a beautiful back-end, but what does it matter to anyone but fellow hackers if a front-end end-user thing doesn't look good, is useful to beginners, but is also useful to those who've been around the block a bit?