The most important use of this logo will be in the clinic's sign. We will be in a retail strip center (example pictures at http://en.wikipedia.org/wiki/Strip_mall). The sign/logo must immediately convey the idea of medical care to someone passing by in their car. Typically, other healthcare companies use the Red Cross cross (either in red or some other color) to denote that, but that is not mandatory. If you Google "medical cross logo" and look at images, you can see some examples)
Secondly, the name Mainstreet was chosen specifically to evoke the idea of small town America. Mainstreet should be done as one word, though right now we are open to either doing it as Mainstreet or MainStreet. Note that the idea of small town America might suggest something -- especially a font - that is ornate, script, pronounced serifs -- but we really want a high degree of readability.
Mainstreet will always be paired with the secondary descriptor of "Family Urgent Care"
In addition to the sign, the logo will be used in all the normal places -- business cards, advertising, stationary, etc. It will also be used a good bit to embroider shirts and uniforms - so there needs to be a simplicity in color and design. Max would be two colors and black.
Key words to describe the brand personality are