I finally looked through a bunch of GENKI 1.
First thing: I've seen a lot of this book before! My Japanese teacher used pages from it a lot during the first couple years. In fact, the first three lessons correspond almost perfectly to the first two years. The last two years were a hodge-podge of things mostly covered between lessons 4 and 9 of GENKI 1.
I actually remember something that bothered me about what GENKI was teaching when I was in Japanese class: very little of the vocabulary taught is applicable to high school life, especially so early on. 14-year-olds don't have jobs or majors, don't buy things all that much, and probably don't go to the bank at all. But seeing it now, it does seem like it's a great selection for college students (the target audience).
Compared to Japanese class, the coverage of how to use grammatical constructs is much more thorough. As an example, like I think I said before, the ～ている form's nuances weren't described in detail in Japanese class, only the specific example of 死んでいる. Luckily, it seems I extrapolated the correct interpretation of why that is, having read the explanation in GENKI. Japanese class tended for focus on specific examples in this sort of manner, with the interesting exception being conjugation. For the ～ない form, we were taught "shift the last syllable from the う line to the あ line and add ない, but turn う into わ rather than あ".
The speed of progression was one big difference I noted. To be fair, a lot of the time spent on the first couple of years was learning hiragana (in year 1) and katakana (in year 2). But it probably would be possible to squeeze more in. Maybe a lesson a week or at least a lesson a month, rather than a lesson throughout the whole year. Then again, I don't know what it's like setting up a high school class.
The grammar dictionary is great. The only minor criticism I have of it is that it would be nice if entries were 五十音 ordered rather than Latin alphabetical, but that's really minor and I understand why they did it that way. I took a look at the entry for ～ように, and I found that it's very clear and detailed. The introduction to basic Japanese grammar characteristics at the start is a treat as well.
Attempting to learn by osmosis is out the window in my book. It is still a nice bonus when it happens, though! I'm starting a new nightly routine: no Internet use when I come home from work, just stretch, study Japanese, and get to bed. (I work evening shifts.) That should get me back into a rhythm of learning, and it also takes advantage of the fact that you remember things you learned just before going to bed better. I'll be using the same tools as before: Tangomon for vocabulary and kanji recognition (it works really well for me), a plain old piece of paper for writing (esp. kanji), and a notebook with a list of current kanji that I can also practice elsewhere with when I'm bored.