There are like 40,000 mods for Skyrim! Some of which are so good, they are "better" than their anologs in the vanilla game. I'm all for a gamer doing whatever they want to do with a single player experience, but just to put it out there: if there is any particular aspect of the game you find "deficient," "annoying," "incomplete," "lacking in aesthetics," "unsatisfactory," or otherwise in need of change, just ask. There are likely at least 3 if not 30 mods that address any given issue.
Not to say that every single thing in the game has been modded and modded well, but many of the main 'quirks' of the game have been, and here is my list of the biggies:
1. It is wonderful that they managed to design a game that runs on console, particularly since the lionshare of their revenue comes from consoles. The downside for we PC users is that we are "stuck" (but not really!) with a horrendous UI that is better suited to consoles. --> http://www.nexusmods.com/skyrim/mods/3863 is the solution. A beautiful mod that completely overhauls the User Interface, while retaining the overall look and feel of the vanilla UI, but adds much more functionality and ease of use for the PC platform.
2. Bugs: it seems that even after the final patch, there were a lot of bugs in the game. There are http://www.nexusmods.com/skyrim/mods/searchresults/? that address these both for the base game and all the DLCs. It seems to me foolhardy to NOT install at least these mods. All of these are available on the Nexus forum, and the Nexus site has its own free mod manager game launcher application that many of the more technically astute seem to disdain (there are many other options too, such as Mod Organizer, or simply using Wrye Bash, which you may recall from Oblivion), but which many thousands of us use and love.
The above two are certainly "optional" but IMHO, they should virtually be regarded as "obligatory" for any PC install of Skyrim. As far as I know none of the above will perform poorly on a lower-end machine. If your machine can run Skyrim, then it should be able to run the above "mods" without any impact on game application performance (somebody more expert please correct me if I'm wrong).
3. Scripting extensibility: http://store.steampowered.com/app/365720/ Many mods use SKSE, and it is in my experience exceptionally easy to install and configure. Many mods that "require" it also offer alternative versions for those who do not have SKSE installed. My understanding is that SKSE is not a "mod" so much as an application which you use to launch Skyrim. If you have it installed correctly and launch it correctly using the SKSE launcher (.exe or .dll or whatever it is) you will not really notice anything different about Skyrim. But what it does is it makes a large body of some of the highest quality mods an available option to you. As far as I know SKSE can run just as well on a low-end machine that can run Skyrim as on a high-end machine, although the more "script heavy" mods you are running will certainly be problematic on lower peformance hardware.
Out of the three, this one is probably the "most optional." But if you do start to consider a few of the best mods, you will want to have this tool installed.