Question

leaf   Some of the leaves of my healthy appearing tree this year have small green to beige projections, almost all on the upper surface.  They are less than a quarter inch long, perhaps the diameter of a sewing needle, and generally project at 90 degrees from the surface.  There appears to be little or no sign of damage, so far, on the opposite side of the leaves.

Answer
This is a gall mite called Maple spindle gall.
Maple Spindle Gall Mites (Vasates aceris-crumena Riley) cause spindle galls to form on the upper surface of maple leaves. During certain seasons the galls are common on ornamental and forest hard maples.
Spindle galls are about 1/5 inch long and similar to a pencil lead in mid-diameter, tapering to both ends. Since the erect galls are conspicuous and characteristic, identification is easy. The life history and habits of this mite are similar to those of the bladder gall mite. The mites overwinter under the overlapping scales of bark in the vicinity of scars and the ends of broken branches. Spindle-galls are extremely variable in abundance.

Although the galls can be unsightly in ornamental trees, you rarely need to treat the condition except for aesthetic purposes, or to protect recently transplanted young trees. The galls do little actual harm. Efforts are best spent on good horticultural practices, such as watering and fertilizing, as needed, rather than spraying.