What dictates whether or not you can give direct, and how do you determine how far down the road you can give direct? Do you just call the next sector, or do you have to ask all the way down the line to whoever owns what the pilot is asking for?
For instance, I fly from ORF to LAS, and the best I can get is a couple states away. When I come out of LAS, I get the VOR that starts the STAR into ORF, almost all the way across the country.
For lower altitude aircraft, a lot of the time a sector will give direct so long as it doesn't conflict with an agreement they have with an adjacent facility. However, we won't know exactly what that adjacent facility has for an agreement with the sector on the other side of them. So sometimes we might give direct, and the next sector could take it away.
We run into this a lot at Waco with a couple of our southern bordering sectors assigning direct to DFW terminal area airports. Everyone has to be on a STAR for the most part, so we have to re route the aircraft back over an arrival VORTAC to join a STAR unless special coordination is affected. That's an agreement WE have with Regional Approach (DFW) though, not the facilities south of us. They have no way of knowing unless they've been around a while, think to ask, or look up our procedures online through the FAA's intranet.
Direct-To routings can cause coordination hassles too. Airspace is largely drawn up around the airway structure. So if an aircraft is on an airway, they're usually bound for a single sector when exiting your airspace. When you send people direct from various points in space, now you might have guys that are exiting your airspace at a point where it borders 2 or 3 other sectors. The hand off only goes to ONE, so the remaining sectors have to get a phone call to approve a point out, since you're now encroaching on their territory as well.
I specified for low altitude aircraft because once you get up into the flight levels, working with centers, I think they have more route management tools at their disposal. They also cover huge areas of airspace as a whole facility, so they'll know that they can't give you direct in this sector, because it'll be revoked a few sectors down the road (under the same facility, or adjacent facility). In the terminal environment we don't look out that far.