I’ve been working on an icon SLD for QGIS for the USGS GNIS database.  As I pulled out all of the categories, I also counted them.  If anyone else just happens to be interested, here are the unique feature classes and their counts in GNIS:

  feature_class  | count 
-----------------+--------
 Airport         | 23202
 Arch            | 720
 Area            | 2557
 Arroyo          | 466
 Bar             | 5870
 Basin           | 4304
 Bay             | 14094
 Beach           | 2409
 Bench           | 724
 Bend            | 2797
 Bridge          | 7356
 Building        | 160291
 Canal           | 21559
 Cape            | 16417
 Cemetery        | 145544
 Census          | 11629
 Channel         | 4014
 Church          | 231967
 Civil           | 64237
 Cliff           | 4479
 Crater          | 246
 Crossing        | 13167
 Dam             | 56931
 Falls           | 2499
 Flat            | 10559
 Forest          | 1314
 Gap             | 8246
 Glacier         | 1021
 Gut             | 3541
 Harbor          | 1271
 Hospital        | 15864
 Island          | 20540
 Isthmus         | 28
 Lake            | 69403
 Lava            | 168
 Levee           | 546
 Locale          | 162518
 Military        | 2860
 Mine            | 36133
 Oilfield        | 4863
 Park            | 69501
 Pillar          | 2092
 Plain           | 289
 Populated Place | 201065
 Post Office     | 66942
 Range           | 2480
 Rapids          | 1062
 Reserve         | 1276
 Reservoir       | 74683
 Ridge           | 15127
 School          | 216473
 Sea             | 28
 Slope           | 373
 Spring          | 38655
 Stream          | 231462
 Summit          | 70614
 Swamp           | 7608
 Tower           | 16800
 Trail           | 11047
 Tunnel          | 750
 Unknown         | 186
 Valley          | 70239
 Well            | 38797
 Woods           | 684
(64 rows)