The majority of Blizzards 1st of April jokes are always on the site and about the games they make, but I can't recall an actual ingame event or change to add to the effect. Although it is very unlikely, as no new items have been added to correspond this, it is still possible!
All critters turn into lvl83 elite bosses, having dragonlike abilities;
All the gear has been changed to look like paladin tier 1;
Your character will, on random intervals, laugh or cry uncontrollably;
Something related to Cataclysm will happen, sweet, and thus 12 million people are trolled by Blizzard when they revert the changes.