Yosemite is one of a handful of US sites that readily meet OUV requirements. It has stunning scenery and has provided an impetus to the overall American (and indirectly world) conservation movement (reference John Muir) and has provided inspiration for artists (Ansel Adams and more). 
It is also the WHS that I know best, having hiked or skied most of its trails over the past 50 years. I am writing this review less as an evaluation than as an introductory guide for other WHS travelers, having appreciated the guidance I’ve found in other reviews for less celebrated sites.

For first time visitors, recognize the great popularity of the park and attendant crowds in many times of the year- all summer, and during the February “firefall” weeks, when, if weather and water conditions cooperate, there is a chance that the ephemeral Horsetail Falls will glow red for a few minutes at sunset. Great when it happens. The park service has used a day visitor reservation system in recent years (both in February and summer) but the crowds can still be a bit much.  

Popular day hike trails can also be crowded.  The deservedly most popular is the Mist Trail up Vernal Falls, and if you have the energy further up to the top of Nevada Falls.  Crowded but great.  I recommend returning via the less scenic John Muir trail to save your knees (a required return during Covid). Having done extensive hiking in all of the US (except American Samoa) national parks, this ranks among the top 5, if not 1, of all day hikes in the US National Park system.  Spring is best when you can expect to get wet, but these falls often continue to run even in drought years.

The Mist Trail is often used by day hikers for the very long, reservation limited, hike to the top of Half Dome. My preference is to backpack to Little Yosemite Valley part way and scale Half Dome from there. Camping on top is no longer allowed. 

Other Valley originated trails include the climb to Upper Yosemite Falls (early season because it can dry up in increasing drought years) or the easy walking Valley loop.   The latter is much neglected, as it is often within sound of traffic, but it has tremendous views, and the upper Valley portions near Mirror Lake are past all vehicular traffic.

A classic half-day hike loop off of Glacier Point Road goes to the Fissures and Sentinel Dome, with 360 views.  A favorite off of the Tioga Pass Road is the trail to Cathedral Lakes, also a special backcountry camping area. 

Best viewpoints by car - Tunnel view (winter or summer) and the view from Glacier Point, one of the premier views in all the country, but only available after the road opens, usually Memorial Day (late May).  Valley view, a left hand pullover off the north Valley Road, provides a classic view of El Capitan and Bridalveil Falls.

Yosemite has threel Sequoia groves, but Mariposa is the largest and most visited so it has a shuttle bus.  I find it particularly beautiful in winter on skis.  

Perhaps Yosemite’s greatest attribute other than the Valley itself are the many miles of fabulous backcountry hiking trails. It is an immense and mostly benign wilderness open to any fit to handle it. The Tioga Pass Road, open only after the snow plows have done their work, provides access to Tuolumne Meadows and other trailheads. There are too many backcountry destinations to list but Glen Aukin and Cathedral Lakes are too excellent high country locations for day hikers or backpackers. Although the Valley can be crowded there are many places of great beauty in the wilderness that offer solitude.

Although one thing or another (Covid, drought, high snowfall, budget) has only allowed the current concessionaire to operate them once out of the last 6 years, the network of High Sierra Camps provide access to overnight hikers without backpacks to some of the finest backcountry scenery. They are highly recommended if they are operating. Although many try to circuit all five, several make excellent destinations if time is limited - particularly Glen Aulin (a relatively mellow hike to cascades on the Tuolumne River), May Lake (the shortest hike in) and Vogelsang (higher altitude).

Best season? When you can go. Summer is crowded and in recent years has been very negatively affected by California wildfires, Autumn has nice weather and high country is still open but low or no water in many falls and streams. Winter is restricted pretty much for most  to the Valley but can be stunning in snow. There is a downhill facility at Badger Pass and some fine cross country skiing if conditions are right (including a backcountry cabin for the hearty). Spring is fabulous- falls are usually full, but high country roads, including the Glacier Point Road remain closed.

There are a range of lodging and camping options in the park. The Valley has tent cabins, campgrounds and several hotels, with the more expensive and grander Ahwahnee one of the park systems’ historic hotels. The historic Wawona Lodge (close to Mariposa Grove) is closed for several years for much needed repairs.

Many visitors are interested in seeing wildlife in western national parks.  Yosemite certainly has wildlife, but visitors should not expect to see large numbers of wild megafauna, as is the case in Yellowstone or Alaska.   I have only seen a lynx once in many years of visiting, and this was during the Covid years when visitation was down and the wild animals reasserted themselves. Black bears were also more commonly seen during that period (strolling along the north Valley Road), but they are also infrequently seen.  Yosemite is an excellent stopover for birders interested in seeing a range of California birds, particularly in late Spring.