"While I can't answer as to what, it is true that War Horse will run almost 8.5 years on the West End. No American play has lasted that long in a while, if ever."
War Horse might not be the best example - it's a highly unusual show, not driven by text, with almost enough music to be called a musical, and appealing to adults, children, non-English speaking audiences, etc. Although technically a "play," that word doesn't really suffice to define it.
8.5 years = 3,536 performances. No play has run that long on Broadway. The longest running Broadway play is still (I believe) Life With Father at 3,224 performances (7.75 years), then Tobacco Road with 3,182, then Abie's Irish Rose at 2,327. More recently, there has been Gemini with 1,819 performances, closely followed by Deathtrap at 1,793 and Harvey at 1,775.