Recs.
Updated
SpecsUpdate
Pros
Pro Venerable engine with a community of seasoned and prolific modders
Bethesda has released their own content-creation tools free to the public since Morrowind (2002), and this tradition has continued through Oblivion, Fallout 3, and Skyrim. Though some aspects of the tools and engine have changed between games, the similarities are stronger than the differences. Because of this, many modders have created content for most or all of these games, refining their skills and design sense along the way. This has also fostered a strong core community of Bethesda game modders, which welcomes and encourages newcomers.
Cons
Con Many mods require external tools and don't support Steam Workshop installation
The Bethesda modding community has overcome many engine limitations by means of a series of Script Extender utilities (one for each game). These are third-party executable applications and not available through Steam Workshop; however, Valve has officially endorsed them and, if present, they will automatically be activated when launching through Steam.
Limitations of the mod file format can cause mods to conflict if they attempt to make unrelated changes to the same object (e.g., one mod changes an NPC's hit points while another changes hair color). Many popular mods have large areas of overlap and will not work together (or at all) if installed through Workshop. Wrye Bash is a third-party application which resolves such conflicts by merging mods together.