Fencing, fencing and more fencing. Viewing areas are generally a long way from the track itself. You can walk along the track from the Porsche curves all the way to Tetre Rouge, but getting to other places (like Indianapolis) requires a shuttle bus.
Each of the grandstands requires a ticket for that grandstand, so general admission doesn't automatically get you in grandstands of your choice. Of course, in the middle of the night, security is less present around the grandstands up and down the front straight.
Forget about access into the back of the pits and paddock without a special pass. Access at US tracks is miles easier. Frankly, that goes for a lot of the areas around the track - hospitality areas are common but only for guests.
I had certain areas that were favorites for viewing - outside of the final curves as the cars launch onto the front straight (Tribune 23), inside of the uphill left hander headed to the Dunlop bridge, and inside of Tetre Rouge. I also really liked the grandstand above pit lane (Tribune 34).
As you might imagine, the place is packed at the start and finish. My favorite time was overnight but don't forget that the night is shorter at LeMans than most other 24h races.
DM me if you need anything else.
