My expectation when I stay at Hilton branded hotels as a DIamond is that there is nothing I can rely on in terms of upgrades. The Diamond upgrade is a space available basis and same time I do think each hotel does set up its own criteria regarding the availability of a diamond upgrade, and I also feel as if it is up to each employee I face when checking in at the front desk.

If I want a specific room type, then I always book that room type, and I never hope for the Diamond upgrade to a better room because the hotel does not look busy the day I am staying. Also, technically the Diamond upgrade is one rank upgrade so the Diamond upgrade can be from a regular room to an executive room whereas for a specific hotel it can be exactly the same room, the difference is a higher floor and any other amenities come with an executive room I get as a Diamond anyway. Also, I never stay at one specific hotel frequently enough to know if there is any difference between my booked room and the room I got. Sometimes the hotel's definition of an upgrade and my definition of an upgrade may not be on the same level. Once I was told that I was upgraded to the larger room, but because of the location of that room in the building the room was slightly larger (by not that much) than other rooms and everything else was the same, the hotel seems to consider that as an upgrade whereas I did not feel that as an upgrade.

When I book a room as a Diamond member, I do expect to stay in the room I booked. I really do not have an expectation before arrival at the hotel that I will get a better room. You mentioned a suite upgrade, if I did not book a room one category below the suite then I will never expect an upgrade to the suite. I am not saying suite upgrades never happen, I have gotten a suite upgrade from the cheapest room available. But if you did not book one category room below the suite, then I do not think there is anything you can rely on for a suite upgrade because I feel as if the DIamond upgrade is more arbitrary rather than there is a proper system in place. If I get an upgrade to a better room then it is a pleasant surprise.