I think "good" at D&D has meant different things over the years. I'll explain...
In 1e/2e AD&D, being "good" at D&D meant you knew what to do in game, and how to survive. Learning to check for traps, and HOW to check for traps, making accurate maps, working as a team (our old motto in 1e was "never separate the party", using strategic hit-and-run tactics, knowing when your resources were tapped, and yes, knowing when to retreat when outclassed made you "good" at D&D. The drawback here was it was easy to fall into a "DM vs. players" mindset (and while that can be a HUGE amount of fun, its not what I really enjoy in a weekly game).
In 3e, this changed- the focus was on building a powerful character and "gaming the system". While it can be fun to mess with all the options and build a character, a lot of people I played with and talked to took it to ridiculous extremes. Now, I know not everybody used the rules in this fashion, but it was the playstyle implied in the core books, and the endless power-up splats that came out. I played quite a bit of 3e with a wide range of groups, and pretty much universally people built their characters from levels 1-20, finding broken combos, and spamming the same tactics/attacks over and over again (trip monkeys, APAATT, sunder, etc). They also tended not to work as a group well, and had almost no notion of retreat, subtlety, or mapping skills. Were they powerful? HELL yes. Were they "good" at D&D? Not in the sense of the 1e/2e sense of the phrase, but they were "good at D&D" for what 3e presumed. For me and my core group, the playstyle 3e encouraged left a lot to be desired.
4e has kind of mixed the 1e/2e aspects of teamwork and smart strategic play with the 3e aspects of character building. 4e doesn't have hardly any of the broken combos of 3e (at least so far), but it does require some system mastery in that you need to understand how to build your character and which abilities best fit his theme and other abilities. The nice part is, you're unlikely to make a sucky character, even if you're a newbie gamer- the system isn't designed with the intentional traps of 3e. But equally (or even more) important in 4e is the party working as a team to overcome challenges and achieve victory, and knowing when to retreat (which happens regularly in the 4e games I run and play). For us, 4e has hit the perfect balance of 1e and 3e, allowing us to have the positive aspects of being "good at D&D" from both systems, while having none of the drawbacks.
And for me personally, being "good at D&D" requires the ability to roleplay and think/behave as his character, regardless of the edition. If someone is playing simply an amalgamation of numbers with the personality of a cardboard box, they can rock out on survival, system mastery, and tactics, and still suck at D&D.