you've misunderstood the "30 seconds of fun" comment. the idea is that if you have thirty seconds worth of deeply enjoyable mechanics, you can then put the player through dozens of situations and level design permutations, and those mechanics will keep it enjoyable. whereas if you don't have those 30 seconds, even the most sophisticated design won't engage.
halo takes a tightly balanced set of core mechanics -- recharging shields, two weapons out of a varied set, enemies with particular though not rigid weaknesses -- and challenges you to apply them to an impressive variety of situations. the variety is strategic, not superficial. the sophistication of the mechanics is such that subtle variations -- often as simple as terrain changes -- can call for hugely different strategies. it's actually subtle enough to miss altogether. halo isn't conspicuously brilliant the way half-life is. appreciation for the original game's campaign took a while to catch on.
this is an oversimplification, but: in traditional action games, there is an ideal strategy, an ideal path through a heavily designed series of obstacles, and players experiment to find that path. if i were an academic, i'd call those games "normative." in halo, the ideal path has been obliterated: the challenges you face don't imply a solution. players experiment to find and express a personal style or styles.
one of the best things about halo is that it doesn't evaluate your performance at the end of a level. it doesn't encourage you to finish levels quickly, or to use your weapons efficiently, or to keep your comrades alive. because halo isn't about normative goals. halo isn't even about not dying, really: it "punishes" death by returning you to a very recent checkpoint. halo is, by design, about experimentation. about testing the limits of richly reactive battlefields. if you think puzzles or hidden doors would improve halo, then you don't understand halo.
and this probably sounds reactionary nowadays, but there is such a thing as objective quality, though our means of recognizing it are imperfect. as such there are games that demand appreciation.
edit: i'll also say that to look for certain design styles or priorities in every game isn't enthusiasm at all; it's fetishism.
