Back in the day, when Burning Crusade was launched, one had to be revered to get their heroic instance keys. This meant you had to grind out reputation with a particular faction to get the heroic key, usually through the highest level instance. This changed in Patch 2.3.0, which lowered the requirement to honored. At that time, there was much rejoicing. Honored reputation with a faction can easily be achieved with leveling quests. You no longer have to cross the threshold of a dungeon to meet this requirement.
The endgame is a progression. Sure, you can waltz into heroics as soon as you're honored with a faction. You can also talk about religion and politics on a first date, but that doesn't mean you should. Toward the end of TBC I saw people in droves trying to jump into Heroics and Raids as freshly minted 70s. Sometimes they had more experienced players along to carry them, but many other times they fell squarely on their faces. WoW can be a vicious cycle where you get better gear to take on tougher tasks to get better gear to take on tougher tasks. The end result is a sense of accomplishment, not to mention shiny Epics (or possibly Legendaries) to show off in you favorite major city.
Here's an extremely simple guide to achieving your PvE endgame goals:
Step one: Learn to play your class. Get to know all of your abilities- what works in the solo environment does not necessarily work well in an instance.
Step two: Learn how to play with other people, this means running instances as you level to 80 and accepting criticism. Somewhere along the way you may find a guild that you click with and matches your endgame goals.
Step three: Now that you're level 80, you need gear from regular level 80 instances. Replace your greens and low-level blues with level-appropriate equipment.
Step four: Run heroics. If you're not sure you're ready for it, ask a group member for a gear check, and accept their feedback. Wipes caused by under-geared players are annoying to everyone. Some may need to return to steps one and two.
Step five: Try your hand at raiding. You might like it, you might not. But trust me, you'll like it better if you complete the first four steps successfully.
Edit: Fixed some confusion over heroic requirements at level 80.