It very likely is true that one person's overlooked national park is another's jewel.
What got me thinking about overlooked units was a newspaper clip the good Professor Janiskee sent me. The clip contained a Top 10 list of sorts of "the most interesting (even overlooked) destinations for 2014," as chosen by the editors and writers of Lonely Planet guides.
This is a head-scratching list of the first degree. Not only is the No. 1 destination the Grand Rapids/Michigan's Gold Coast (perhaps that's the overlooked apect), but the Jersey Shore was No. 5, two rungs ahead of Cumberland Island, Georgia (I'm assuming they were referring to the national seashore there...), and five slots above Lanai, Hawaii!
Now, I grew up vacationing on the Jersey shore, and I would much rather visit Lanai than Red Bank.
That said, here is one Traveler's list of the top 10 most interesting/overlooked units of the National Park System:

Owachomo Bridge, one of three natural bridges at the monument. NPS photo.
1. Natural Bridges National Monument, Utah
This small (~7,600 acres) park in southern Utah no doubt gets overlooked due to the nearby presence of both Canyonlands and Arches national parks. But it's a gorgeous unit that shouldn't be missed. The night skies here are incredible, the small campground (13 sites) intimate and crowd-proof, and the geology and archaeology fascinating.
2. Saint-Gaudens National Historic Site, New Hampshire
Covering fewer than 150 acres, Saint-Gaudens easily could be overlooked in its location near Cornish, New Hampshire. But if you treasure the arts, you need to stop by. Here on 148 acres is the home, studios, and gardens of Augustus Saint-Gaudens (1848–1907), who was considered to be America’s foremost sculptor of the late 1800s and early 1900s. Six historic buildings are open to the public with more than 120 original sculptures on exhibit.
3. Channel Islands National Park, California
Off California's coast, this national park no doubt is overlooked by many because 1) you need to take a ferry ride to reach it, and 2) there is no lodge to call home. But the five islands that make up this national park are unique in their remote setting, contain curious and interesting flora and fauna, and boast some of the world's largest sea caves and rich marine life.

Blue tangs swarming at Buck Island Reef NM. NPS photo.
4. Buck Island Reef National Monument, U.S. Virgin Islands
Even more remote than Channel Islands National Park -- you need to fly to the U.S. Virgin Islands, then take a boat to reach this national monument -- Buck Island protects what is thought to be “one of the finest marine gardens in the Caribbean Sea."
5. Black Canyon of the Gunnison National Park, Colorado
With fewer than 200,000 visitors in 2012 (most recent year data are available), you won't experience crowds here. But you will find some of the steepest canyons in the United States, a raging river running along the canyon floor, two nice campgrounds, and some very, very rugged hiking.
6. Devils Postpile National Monument, California
This geologically, and geograpically, unique park unit gets passed by visitors more interested in skiing at Mammoth Mountain or reaching Yosemite or Sequoia national parks. But it's a jewel in the rough, what with its pile of posts, peaceful and beautiful meadows and stream, and the John Muir Trail that passes through.
7. Golden Spike National Historic Site, Utah
Who wouldn't be interested in the history of Western expansion in the United States and steam-billowing locomotives? The monument's visitation stood at 42,551 in 2012, not a gold rush of any sorts, but still ahead of such notables as John Muir National Historic Site, North Cascades National Park, and Isle Royale National Park.

This fully-articulated early horse is the only horse fossil found in the Green River Formation. Notice the fish fossils near it. NPS photo.
8. Fossil Butte National Monument, Wyoming
Located in western Wyoming, Fossil Butte's overlooked dilemma no doubt stems from its location. I lived in Wyoming for nine years, and in Utah for 20, and haven't come within 20 miles of the monument despite my many travels. But those who have made the trip have found incredible fossil resources, and felt the wind rushing across this Plains landscape.
9. Thomas Stone National Historic Site, Maryland
Home to a patriot, this residence 30 miles south of Washington, D.C., sheds some light on 18th century life as well as on a man who realized the Revolutionary War with England was inevitable. Thomas Stone went on to become one of the 56 signers of the Declaration of Independence. His grave can be found in the family cemetery on the site.
10. Aniakchak National Monument and Preserve, Alaska
With just 19 "recreational visitors" last year, it's safe to say Aniakchak is hard to reach, but overlooked? Perhaps, but it's definitely interesting! "The monument is home to an impressive six-mile wide, 2,500-foot-deep caldera formed during a massive eruption 3,500 years ago," notes the Park Service.
Comments
May I nominate Hovenweep in southeastern Utah?
I'll second the nomination of Hovenweep.
I'm tempted to throw Yucca House NM in here, but I suspect that would be cheating .
I nominate Hopewell Furnace National Historic Site in Pennsylvania.
Coincidentally my screen saver right now is a photo of the Natural Bridges National Monument entrance sign I took in June, 2013. I also visited, among other National Parks, Hovenweep and Yucca House. The visitor's book I signed at the entrance to Yucca House had only four or five other signatures in the last four years. Beautiful places...
I could go on and on and on but here is my short list:
Chaco Culture National Historical Park- Probably not overlooked but definitely undervisited given its remote location.
Thomas Edison National Historical Park - Just really cool
Andersonville National Historic Site - Incredible Museum
Manzanar National Historic Site - So moving
Hovenweep National Monument - Everybody see the trend? This is one of the best little parks!
John Day Fossil Beds National Monument - Painted Hill unit is incredible
All great suggestions. Hovenweep really is awesome. It's uncrowded. There are a variety of sites to visit, so I've had something new to see each of my last few trips there (and I still have Cutthroat Castle to see for the next time! And it has an uncrowded, relaxed campground! It's the only place where I've been the only attendee to an evening program, and mind you, that was 100% attendance that night!
I also would add:
Tonto NM
Gila Cliff Dwellings NM
Big Hole NB
Grant-Kohrs Ranch NHS
St. Croix Island IHS
Friendship Hill NHS
Salt Creek Massacre NHS
Fort Bowie NHS - Arizona
Allegheny Portage Railroad NHS - Pennsylvania
Fort Washington Park - Maryland
Ninety Six NHS - South Carolina
Agate Fossil Beds NM - Nebraska
Fort Union NM - New Mexico
Chiricahua NM - Arizona
I will second Gila Cliff Dwellings and nomiinate Navajo NM. Let's not forget Dry Tortugas, Booker T. Washington, and Maggie Walker..
Rick