Mods with more scripts running at once will take more power to run; and those grass mods. If you have any sort of modern up to date non-cheap computer(as in not netbook and with a dedicated graphics card), you should be able to run anything.
Agree.
Lots of mods that have heavy scripts running constantly can cause some slow down, but it's not too big of a concern for the most part. Most of these mods will use MWSE.
Animated grass won't be feasible on low-end machines.
The more NPCs you add, the more of a demand they place on your computer. NPCs use a lot of memory when being rendered at once.
All the fancy shaders, waters, and Distant Land of MGE won't be able to run on low-end machines.
High-poly count meshes will really slow you down. There are not very many of these, though.
Other than that (unless I missed something) you are pretty much free to add all sorts of stuff without worry.
There are also tools to help optimize performance.
Like the http://planetelderscrolls.gamespy.com/View.php?view=utilities.detail&id=22 and the http://www.ntcore.com/4gb_patch.php.
There are two unofficial patches as well, (Code Patch and Patch Project) that fix many things and make the game more stable. Info on them here under the Essentials: http://www.gamesas.com/topic/1305729-mod-recommendations-for-new-players/