Mearls really is a good game designer, and could become one of the greats. His stuff really appeals to me. Iron Heroes is a great example, but his various adventures and Dragon articles over the years really laid a foundation of coolness in my book. What cemented my fanboyishness was two books - the Book of Iron Might, the precursor to Iron Heroes, and the Psionics Toolkit. The Psionics Toolkit was the more significant of the two, simply because I had no real interest in psionics prior to it and this book actually made me want to use 'em in my campaign world.