It would have been easy enough to explain away Vulcan's and/or Earth's paucity of defenses.
Vulcan was clueless. They thought it was only natural phenomena. Earth didn't have defenses because Nero had the defense net codes after torturing Pike and they had already sent their ships to be destroyed earlier. Those points were mentioned in the movie. If you didn't like that story, that's up to you, but saying it was "lazy writing" is an odd argument because these points were actually explained.
Im sorry, I agree with the "lazy writing" analysis. See, I think that the writers tried to jam too much back story into a short 2 hour film. Personally, I think that certain things did need a more elaborate set up and a much more in-depth development. For instance, if it is true that they had already sent their ships to destroy Earth's defenses, a quick 1 minute shot of those ships attacking Earth's defenses would have cleared that right up. That said, since when does a mining ship have enough fire power to take out EARTH's defense net?? One little mining ship would never have been enough for that.
Remember, we're not bashing on the movie just to bash... we're doing it because we care and want the writers to fix it the next time around (and believe me, I DO hope there is another one). Through all its faults, this movie did excite me about the franchise again and I'd be more than happy to see the next one. All I ask is not to have my intelligence insulted by one of my favorite series, thats all.