Other considerations/ideas:
1. Have a Guild control the portals (ala Dune). Only the guildmasters can open portals and no one (absolutely no one) is willing to tick off the Guild for fear of having its portal shut down. This neatly avoids the problem of kingdoms using the portals for warfare. Versus terrorists, e.g., a mad lich, the cities of the world would probably have a policy of crushing anyone/anything that messes with portals or those who can operate them.
2. How far down into society do the portals go? To small cities, or only large ones? To major resource sites (mines, forests)? Do non-human realms allow portals? If so, which ones? What controls do they place on portal use, besides those the Guild impose?
3. How high the Guild sets fees will determine who uses the portals. If you still want a lot of regular travel, set the fees higher than the average commoner and merchant can afford. If you want the portals to be the normal means of travel between cities, set the fees very low, or have a sliding scale based on what is transported (people are cheap, goods more expensive). Remember, the portals won't go everywhere, so normal travel will still be required between points without portals - the portals could in effect be similar to railheads during the 19th century.
4. If you use a Guild or something similar, then politics (again ala Dune) could be very important. Perhaps growing cities vie over becoming the next site of a portal. Or a kingdom offers the Guild extra fees, if the Guild will build portals to particular sites within the kingdom (or deny such portals to rival kingdom). The Guild will have to appear impartial, but is that only appearance?
5. Or, alternately, have the portals be fixed. They were created in ancient times and cities with portals have prospered over those that don't have them. The Guild could still exist - it's the only group that possesses the lost art of using the portals. In this situation, you'll likely have other, newer, cities without portals and a strong rivalry could develop between some of them. The non-portal cities would likely band together to oppose those with portals (since they would have an "unfair trade advantage") or to put pressure on the Guild to keep the portal cities from having too big an advantage. Also, imagine how lucrative it would be if someone else discovered the secret of the portals, breaking the Guild monopoly...a lot of folks would be working hard to do so. As you can see, it all starts getting rather complicated, so it really depends on what kind of campaign you want.