The things I like best about Marriott:

1. At their hotels & suites, the concierge lounge offers a very good selection, better than most full-service hotels I've personally stayed at. Ex: the usual fruits and cheese platters, free sodas & juices, but also serious hors d'ouerves like sweet and sour chicken, jambalaya, fried shrimp, and baked ziti. I don't even need to order dinner unless I happen to be exceptionally (for me) hungry. And, if you're platinum level, access is free.
2. The free gift boxes awaiting me. Every week: bottled water, chips, salsa, dried fruit packs. Plus chocolate truffles every night (NOT equal Rudi's Swiss chocolates, of course! OMNI: Thanks again, Rudi, the whole family really enjoyed those you sent.)
3. As the others have said, availability and consistency. Most towns, there is some Marriott property available, and you know what you'll get. If you're going to use some points and book a free stay, there's probably some kind of Marriott property nearby.
4. The Residence Inns' free shopping. Just fill out the shopping list on the counter and the groceries'll be in the fridge when you get in that evening. VERY nice service when you're in a strange town! There is no fee for the service and the groceries are added to your bill.
5. Electronic checkout at the full service hotels. The morning you're checking out, your copy of the bill is slid under your door. Turn on the TV and check out, or call the front desk. No waiting in line!

Marriott isn't perfect, but it is my favorite chain.