Some things are not available. You can not make in-game purchases to customize your gear or camp. Users also note that you can not play card games, so you'll want to forget about cleaning up in a round of poker. So, do not be surprised if things substantially change during the beta phase. Rockstar noted that there will be "additions and updates" during the test, and warned that there could be rank or stat resets if there are problems.
As to what it's like to play? To some extent, it will feel familiar if you have tried GTA Online… and not always in a good way. Early players have observed a lot or aggression, and it's the point where you can generally assume that people outside your posse are trying to kill you. This is not the large-scale multiplayer Western game of your dreams, at least not right now. With that in mind, Rockstar is asking for feedback – the experience could change as the developer figures out what works.