The Wildlander wiki does a much better job of summarising the modlist than I could:
But I'll try to summarise-the-summary a bit here...
Basically Wildlander adds:
1) Requiem in a tweaked format that includes Minor Arcana and Dragonborn patch
2) Alternate start support (skips initial tutorial section of Skyrim)
3) A tweaked character creation system which allows for more diverse options
4) Vastly improved combat using Wildcat and Mortal Enemies as well as many other mods
5) Survival mods so that you must eat, drink, sleep and manage your temperature to survive
6) New hunting mechanics from the Hunterborn mod
7) A modified crafting system using CCOR which was very time-consuming to set up
8) A completely new UI using a variety of UI mods and including a custom-built sneak meter
9) A dynamic economy and a variety of 'jobs' to perform in the game world
10) Realistic penalties for committing crimes, i.e. the gods stop you from receiving their blessings
11) More realistic factions, such as the College and the Thieves' Guild having actual requirements to join
12) Spell Research and new spell visuals - SR gives you many new ways to improve your spell-casting
13) Disabled fast-travel and thus the motivation to travel via carriages/ferries, and new horse features
14) A comprehensive suite of follower mods making them less shit (no Inigo yet, though Lucien is coming soon)
15) Population mods making Skyrim's world less barren from a people perspective
16) Player housing expansions
17) Quest improvements ranging from small to quite big
18) Immersive nights, conversations, names and other features
19) A variety of miscellaneous mods such as GIST
20) FINALLY, a much improved camera and much better third-person camera support (I play 1st-person myself but there you go)
Damn, it's a lot.
Note that this list is actually kind of lacking. It's more of a list of "core features." Just as an example, in Wildlander you can actually build your own town, complete with features like transporting certain NPCs to that town and having them go about different kinds of business, have different schedules and stuff like that. I built my own town like that around the honey farm home when I played last time.
There are multiple "advanced features" in the modlist on top of the stuff Yosharian mentions, which are features you can completely ignore or use at your leisure.