I can tell you, as some one who has played since Vanilla, that the instances even back that weren't necessarily "hard" (well, not usually). It just required a little planning before action. Just take things one step at a time and you will fine. It will go something like this: "Ok, cc the guy on the left, we focus on the dragon guy, and then kill the ogre. Once that is done, we will finish off the cc'ed target." So long as every one does their job, things should work out pretty well. There will be more dangers to watch out for, but in general, as long as you pay attention to what is going on, you should be just fine.

Now, from what I have seen of some of the heroic encounters, there WILL be some pretty damned challenging material but the concepts remain the same. Do you job, pay attention to your surroundings. Gear wont be as big a problem as you might think also (unless you are a tank); even if you are a little lower than the suggested gear level, you can usually pull through so long as you use your head a little. A slightly under geared dps will out damage a dead dps no mater his gear.