If you are planning on going, I recommend looking at the map and hotel sites now and making your decision early, and probably booking as soon as you can.
The Party Hotel is not the Closest Hotel. In fact, the hotel where parties will be located (the Davenport, which apparently is to be reflagged as a Marriott before Worldcon; Mariottnishis take note) is the hotel farthest away from the Convention Center. (About 1 km/about half a mile/slightly less than the distance the Loncon convention space was from the Crowne Plaza.)
There are a pair of Red Lion Hotels on the opposite side of the river (nearby pedestrian and road bridges link the sides of the river) from the Convention Center that are closer to the CC than the Davenport; however, they are both on the opposite direction than from the Davenport, so if you stay at one of the Red Lions, you're facing a longer walk to the party hotel.
The hotel closest to the Convention Center (indeed, it's connected to it by skywalks) is the Doubletree by Hilton. While Sasquan is, I'm told, retaining blocks of rooms for people with disabilities, if you are someone with mobility problems who wants to book through the normal process and not go through Disabled Access, or if you consider proximity to the Convention Center your highest priority (it is for me due to my commitments to Sasquan), I suggest you get your reservation for the Doubletree in at the earliest opportunity. Personally, I expect the Doubletree will fill up the same day that reservations open.
I do not speak officially for the 2015 Worldcon in Facilities matters. You cannot take my word as official on anything having to do with Sasquan Hotels. I'm just another fan, albeit one who is an Area Head for the convention and will be working hard on it.