One more point of anecdata: I've been with my current employer for just over 14 years, and I've worked on probably about that many different projects--generally two or three at the same time--in a variety of roles and with a variety of technologies (often with some degree of leadership, both formal and informal). There are many aspects of my current situation that I appreciate, but the plethora of options available for me to work on certainly feels like one of the big reasons I've stayed for so long.