What happens if I do this? (WoW)?

Your friends was exactly right you will be able to play with people with cata/wotlk but you can't travel to the Northrend or Infralar. And you can't level past 70 without WOTLK then 85 for CATA. Yes you can do achievements they only differnert is you can't level past 70,quest in the newer areas, and buy flying mounts in Kali/Easten K