Complex question I will try to make simple:

1. Free nights with points, with a reasonable rate of redemption relative to spend. For me, that is by far the #1 reason to participate in hotel loyalty programs.
2. Marriott is very strong on the upper end. So I stay for business in mid-level (FS or Courtyard) and vacation in JWs or Ritz, using my points.
3. Potential for upgrades once achieve Gold or Platinum. Marriott does an average job in this regard. Better than Hilton, worse than Hyatt.
4. Free breakfast--except at resorts and Courtyards in the case of Marriott Rewards (Marriott Rewards' biggest weakness).
5. In the case of Marriott--they are arguably the most consitent of the major chains. Walk into a FS Marriott and you will get a clean, green carpet in your room. Guaranteed. Etc.
6. Lifetime elite status. It's a biggie (hullo Hillton HHonors!).
7. Very good likelihood of a Marriot family of brands hotel wherever I need to be, and often several. Only Hilton does better in this regard.
8. Marriott Rewards customer service is very good at all levels. Contrast that to the Hilton Diamond Desk--pathetic on a good day.
9. Also own a Marriott Timeshare--we've had great expereinces, successfully exchanging within the Marriott system every time (have never used our native property).
10. Did I mention free nights with points?!!!!!....