Power users know that, when you're composing text, it's often easier to move your cursor around (and potentially highlight chunks of text) using the keyboard, instead of moving your typing hand over to the mouse. Most Mac apps support something called Cocoa key bindings, a fancy term for system-wide keyboard shortcuts for navigating and working with text. Some are probably familiar to you, such as combining Command with the arrow keys to move the cursor within the current line or document (or Command-Shift-arrow to select text). There's also Option-left arrow and Option-right arrow, which move the cursor through your text word by word. Others are less well-known: Control-A, for example, moves the cursor to the beginning of the current paragraph; Control-O splits the current line, inserting a return without moving the cursor to the new line; Control-T transposes the two letters on either side of the cursor.
Daniel Jalkut, tech blogger and founder of Red Sweater Software, noted on his blog that Lion tweaks the behavior of those Option-arrow text navigation shortcuts. In Snow Leopard, Option-arrow treated certain punctuation-separated strings (like.this.one) as separate words--meaning you could Option-arrow navigate between each of those three words. Lion, on the other hand, treats such strings as single words. Lion's change may well make sense for you. But for programmers, or for people who type (and edit) URLs often, the change may be less welcome.
Fortunately, Jalkut discovered a straightforward way to revert Lion's keyboard navigation behavior to match Snow Leopard's. Launch System Preferences and select the Language & Text pane. On the Text tab, take a look at the Word Break option on the right side. Lion's default selection is Standard; to emulate Snow Leopard's behavior, change that to English (United States, Computer). (Jalkut's commenters pointed out that the Word Break preference was introduced in Snow Leopard, but it seems that Lion changed the default to Standard.)
Once you've changed the preference, you'll need to restart applications to get them to take advantage of your new setting. And note that certain applications--including BBEdit--assert their own preferences for some key bindings, so making the change above may not affect them at all.