A lever is a binary option - you can either use the rules for marking, or you can not use them. A dial is something with multiple settings - a long rest can take one hour, or eight hours, or one week.
There are definitely more levers than there are dials, though, and most of the levers control things that will never come up either way.
Hmmmm, I've always thought of things that were binary as switches, not levers.
To me, it was:
1) Plug-in - Independent bit of rules that doesn't replace other rules (typically), but merely adds complexity to them and doesn't really consider other plug-ins, levers, etc.
2) Switch - A binary option - either this OR that. Either MP OR spell slots, for example. Either single-classing OR multi-classing.
Switches and plug-ins are often very similar and can be hard to delineate, so I typically call them "plug-in" thanks to FUZION and the like (was it FUZION? Something of that era, I suspect). There's also definitely a question as to whether a lever with only two options is a lever or a switch. I think if it easily could have more options, maybe it is fair to call it a lever. I do think it has to replace rules, rather than add to them, to count as either, though.
3) Lever - A set of choices, of which you could only select one, but which might all be completely different in nature (a 5E example is XP gain methods - you can only really have one, but they're just totally different beasts, not faster/slower).
4) Dial - Something which was a matter of degree, not so much of different choices. An example would be XP gain RATE as described by Lanefan below. Another might be the "natural" rate of recovery from injuries. Or as you say, the long-rest duration.
I'm not saying you're "wrong", just that my understanding of the usage of the terms was different. 5E certainly does have levers and dials, just less of them than I'd expected.
You can also create what might be called dials or templates by packaging other optional rules, which is what I was surprised to see was missing in 5E.
I was hoping the basic/standard/advanced would be used to frame the options is a more coherent way - but it seems given the WOTC's limited release schedule, this is not going to be the case.
Indeed, and that's rather disappointing. It doesn't seem like we'll see any actual rules-oriented books ever again (which will cheer some, I'm sure, but every decision cheers someone!), which would be the primary ways to deliver coherent options. If it's all going to be piecemeal "AP + AP-related playerbook", it's likely to remain very incoherent.
