A common misconception about downtown areas is that great things can be achieved by "shuttles" or "circulators," short routes that just run around in downtown. The problem with these lines is that they save time for the customer only if they are very, very frequent. This is an issue that separates people who need only a symbolic service (such as a line on the map or a photo of the transit vehicle in front of the development they're trying to sell) from people who want actual mobility.
Frequency is really important, but it's also really expensive. Doubling the frequency of a service (i.e. halving the "headway" or elapsed time between consecutive trips on the line) comes very close to doubling its operating cost. If you double your peak frequency (i.e. the highest frequency you run) it also doubles your fleet, which doubles your fleet capital cost, your ongoing mainteance cost, and the size that your storage and maintenance facility needs to be. So it's not surprising that we see a lot of downtown shuttle services that offer a line on a map, and a photographable vehicle, and even some mobility for people who aren't in a hurry, but that don't really compete with walking, at least over part of their length.
For an example, let me pick on a very credible-looking route from a proposed downtown Oakland (California) streetcar network, from the interesting Bay Area blog 21st Century Urban Solutions, by Daniel Jacobson of Stanford. Daniel has lots of good ideas, and I recommend his blog, so I hope he won't mind me using his idea as an example.