I think it's missing a level (SL0.5 - linear)

all levels above 1 assumes that the compounding effect of technology will continue into the future, but it's also possible that this growth will level off or come in spurts as major advancements are made, and that the current exponential growth is simply our civilization moving from one plateau to another.

At SL3~4 some of the ideas also assume that the space of possible technology/intelligence is a smooth continuum that extends to infinity. However it's perfectly possible (likely, in fact) that faster-than-light travel is simply a physical impossibility, or that there is a ceiling on intelligence which is a very ill-defined quantity to begin with.

Bearing that in mind, here is my slightly less optimistic prognostication:

Technology will reach a plateau as the low-hanging fruit borne of the scientific method is exhausted. Progress will still be made linearly, with each year bringing less change than the last. Although the total number of minds working on technological problems will increase, the mental effort required to reach the state of the art in very narrow disciplines will approach one human life time. New and exciting technology will be produced, but it will often be the application of existing technology to a new field rather than the result of fundamental research. This state continues for several hundred years, after which linear progress has produced significant results in human lifespan, intelligence or both via genetic engineering and all bets are off.

That put some things into perspective. Thanks for posting that. I've been pretty much at SL0 (slightly into SL1, been considering VR a bit) with my thinking and personal projects, I realized, when I should be at least SL2 (not having a term for it), at least as far as my thinking, if I'm wanting to make a long-term impact. Might not go quite as far as RAW did, though, even though I like him a lot.

SL2 tech: major genetic engineering, medical immortality, interstellar travel, and new “alien” cultures

SL3 tech: Immortality, nanotechnology, human-equivalent AI, intelligence increase, uploading, total body revision, intergalactic exploration, megascale engineering

I think we're making progress toward SL3 tech as well as SL2 tech. Things like interstellar travel and "alien" cultures seem off (unless you could today's culture "alien" to the past). However with recent work in nanotechnology and AI (even megascale engineering), we can already see ourselves working on SL3 tech today.

That link was quite the fascinating read. Here's an open question, but do others experience some form of anxiety when thinking about future developments such as this?

It's not out of fear. To me, I want to truly experience the advances, not just perish and be forgotten in time. Science is so blissfully fascinating; there's so much yet to learn.

Perhaps a better way of putting it is a realization of your own mortality?