Here are the main highlights; see the release notes for a full list of changes.
Full 64-bit support
You can now make Stardew Valley 64-bit on Windows with an unofficial patch, which removes limits on how much memory the game and mods can use. That means you can install far more mods (or bigger mods) without running into dreaded 'out of memory' errors.
SMAPI 3.10 adds full support for 64-bit mode, including in the installer (which detects which version of SMAPI you need) and update alerts (which check the 64-bit game patch version if it was applied).
64-bit mode is optional: SMAPI still fully supports the normal 32-bit version of the game.
Better 'skipped mods' groups
SMAPI now has smarter groups for skipped mods, so it's easier to see which mods to try updating first.
For example, players often have hundreds of skipped mods if Content Patcher is broken. Now it's grouped at the top instead of being mixed in with the content packs that need it:
SMAPI now detects and recovers more game errors (like crashes when a map fails to update seasonal tilesheets), and better handles mod errors during asset propagation to minimize their impact.
For map mod authors: ignored local tilesheets
When editing maps, mod authors often copy vanilla tilesheets into the map folder to avoid errors in Tiled. Then they need to delete the files before testing the map, since otherwise SMAPI will load the local tilesheet file instead of the vanilla one.
If the tilesheet filename starts with a dot, SMAPI 3.10 now ignores it and looks for an asset without the dot:
See Modding:Maps#Local copy of a vanilla tilesheet for more info.
Other improvements and fixes
See the release notes for a full list!