First logins and slow machines present a slow rez.
Sometimes you just need to stay logged into a place for several minutes before all the assets are cached on your machine.
Same with other avatars, you might be standing right in front of an avatar but nor see him for a few minutes, suddenly he appears, and the scene is the same, it will eventually appear.
Domains often seem to just disappear, but they come back, I think there is some network and port switching going on and you will see this a lot, if you dont see “Not Connected” at the top of the interface near the domain name you can assume the content will eventually rez for you, then next time you go there it will be faster to load.
Finding someone else inworld, check the domain you are in, and find a user logged in to a different domain, then click his avatar name, you should get moved to right in front of him, check the top to see if you actually went to his domain. If you did, just hang there for a bit and let the surroundings and avs rez up.