Location, Location, Location. That is a big determining factor of the loops status. Plus the size of the concrete pads as well. Oh yeah, and $$$.
The location may or may not be a factor depending on how you want to get around. If you have a golf cart (or bikes or something similar) the location is less of an issue. If, however you are walking or taking the bus everywhere, well then this could be a factor.
As far as reserving a loop, the advice to note your reservation is good advice, but I would also add, to follow up with a call to them about a week before arrival. That seems to be when they work on assignments. Also as you check-in, if you come in Earlier in the AM, you have a better chance to get your preference as opposed to later in the day. If someone comes in before you and there is an issue with the site they were assigned, (They don't fit, don't like or whatever), they may get re-assigned. This means that your preference could be snagged up before you get there. No site is guaranteed until it's actually assigned, so it really is a first come first serve.
I know for us (we have a 38' TT) we have gone a couple times and had to go back up front to get a new site. One site they gave us, we couldn't get in even if we had a large shoe horn

. We have since gone around to the loops and looked at the sites for size, angle, location, etc, and have developed a list of the sites we know we can fit into without too much issue. That way when we check-in we have a large selection that we can work with. Saves us many trips back to the front.