Well, here's the most important ones:
Swipe left on the blue bar to move cursor to the left, swipe right to move to the right. Swipe up or down to quickly move between lines.
Double tap on the blue bar to start word selection. Double tap again to cancel.
Tap spacebar twice, and the '.' appears.
Shell and space at the begining of the sentence auto expand into She'll. isnt automatically expand into isn't. Other common contractions work the same.
If you want to type one or two numbers don't tap on Number or Symbol button. Touch it and slide your finger to that number. Then let go. You will switch back to original keyboard immediately.
Many iPhone's keyboard buttons have special characters hidden in them. Just press and hold the letter and you will get a popup with all the options related to that key. For example: List bullet (•) is under dash (-).
Shake your iPhone like you are furious. You'll be given an option to undo what you did, and redo what you undid.
Swipe to the right in Letterspace to go back to previous page. While the keyboard is showing up, swipe from the edge of the screen.
Press and Hold anywhere in Letterspace to starts a new note. Press and hold any entry under mentions or hashtags to start a new note with that entry.
Use #HASHTAG like you are shouting. Your notes will stay in that tag even after you have archived those notes.