Not all mods made for Skyrim LE will work out-of-the-box in Skyrim SE. A fair amount of them will need porting.
Plugins created for Skyrim LE must always be ported. This requires the Creation Kit and sometimes SSEEdit for additional steps.
Check the Porting SLE Plugins page for instructions.
Archives (BSAs) created for Skyrim LE must always be extracted. They will otherwise crash Skyrim SE on launch.
Please consult the article on Extracting BSAs for instructions.
Textures created for Skyrim LE sometimes need to be compressed or changed to a different format for Skyrim SE.
Since Skyrim SE, the engine supports the BC7 compression format which combines the small file size of a compressed texture while retaining nearly the same quality as an uncompressed texture of an older format. Textures for Skyrim LE were sometimes left uncompressed for better visual quality, but should be compressed to BC7 for use in Skyrim SE.
Textures can be processed with CAO for optimisation. Instructions can be found here.
Meshes created for Skyrim LE often need to be optimised for Skyrim SE. They may crash the game or cause visual issues otherwise.
You can use either Cathedral Assets Optimizer or SSE NIF Optimizer for optimising meshes. CAO includes NIFOpt though the latter still has some useful additional options that cannot be accessed through CAO (e.g., more detailed logs and removal of parallax).
Instructions for processing with CAO can be found here.
Animations created for Skyrim LE need to be optimised for Skyrim SE.
Animations can be processed with CAO for optimisation. Instructions can be found here.