OK, this has been sitting around as a draft getting stale...
In his GenCon blog, James Wyatt wrote, concerning running Dungeons & Dragons 3.5 after having spent so much time developing the 4th edition rules:
Oh, look at all the people forgetting about attacks of opportunity (especially at reach) and getting pummeled as a result.
This seems like bad form to me, no matter what system you’re using. The DM should give players fair warning of possible attacks of opportunity. Attacks of opportunity aren’t traps; rather they are calculated risks. Arguably, the whole point of the rule—more often than not—is to prevent the actions that provoke opportunity attacks. “Threatened squares” represent a very obvious threat to the character. If that threat isn’t obvious to the player—because a miniature on a battlemat doesn’t fully convey the threat or because the player is less conversant in the intricacies of the combat rules—the DM should warn the player in advance rather than surprising them with an attack.
(Not to say that there might never be a suprise opportunity attack, just that it should be the exception rather than the rule.)
Which is part of a larger point that applies beyond attacks of opportunity and beyond D&D3.5. The GM should always give players fair warning. Might the player not realize the risks of that action as much as the character would? It’s better to warn the players too often than not enough.
(None of which is not meant to be an attack on James by any stretch. Just an observation spring-boarding off his post.)