As simply as I can put it, all Nova does is replace TW as a pure launcher but with more options and settings. Once installed and set to be your default launcher, it's up to you what you do with it, and how deep into it you get. Just like when you first get your phone, you have to set everything up with TW ... well, you have to do the same once you install Nova.
That's all you are really hearing about people downloading stuff to "customize" it.
Point being, if you want ANY widgets other than what comes on the phone, you have to DL them and set them up - same for TW or Nova ...
You want custom icons? You have to DL and install them - same for TW or Nova ...
so on and so forth.
Long story short, if you like TW and don't care about gestures, more dense home screen grids, being able to use custom made icons YOU make to replace what you see on the home screen, etc... then there is NOTHING wrong with TW.
for those of us who want either a little more or a lot more customization, well ... there are third party launchers, like Nova.