For this post I'll simply link to an article from Dungeon's Master, The Golden Level of Heroic Adventure. Though I would disagree that level 12 is a sweet spot (there's a greater power jump at 11, which is when you finally get to experience Paragon tier), I think that the author is otherwise right on the money. I'd add that the sweet spot starts at level 5 when you get your second daily, peaks at level 6 (which as the article mentions is an important feat level and you get a utility power, which is one of the most customizable elements in the game), and continues into level 7 (having three encounter powers increases your offensive capacity, particularly nova or alpha-strike in many cases, and boosts your overall versatility for many classes). Level 8 gives those PCs who started with even abilities (which is probably most of them) their first numerical boost, and the feat is certainly nice, but this is where you start inching toward Paragon tier and the challenges become much more difficult before you're fully equipped to deal with them. Status effects begin to creep up more often, and many fights pit you against Paragon level monsters (level + 3).
I think the most important issue that the author touched upon, however, is that as a DM you should use these sweet spots as benchmarks for your campaign's pacing. Something should culminate within the level 5-7 range, and after that a more difficult challenge should rear its ugly head. I've found late Heroic (levels 8-10, but especially 10) to be almost as challenging as early Heroic precisely because of the Paragon level threats that the Heroic PCs will face regularly. Using the same encounter design principles that you've been using (perhaps throwing a little more at the PCs since they're gaining power) will result in the PCs feeling slightly overwhelmed without much effort by you as a DM. Make sure that the plot matches this increase in difficulty, and then have the PCs finally reach a major turning point at level 10 or 11. At level 10 the PCs will be struggling by the skin of their teeth to defeat the final "boss" encounter. At level 11 you have a couple of choices as the DM; either use the power boost as a narrative element corresponding to a major advantage that makes the boss fight easier (showcasing the new power of the PCs), or take advantage of the PC's newly-acquired power to go all out with the boss fight (something that the PCs would be unlikely to have survived in Heroic tier). Either way, it pays to be aware of these sweet spots as you try to set your campaign's pacing.