...and as a followup, how much should I care?

I've always used krausen presence as a rough indicator of fermentation vigor in primary. But my current batch hardly has any, with a few days at never more than a half inch. It's definitely going, with a bubble every few seconds from the airlock. However, it was a slow start (almost 24 hrs before activity) and it's never felt like it was going too strong.

But it got me thinking: what exactly is krausen, and how reliable an indicator is it of fermentation health?

As a matter of interest, this is what I did:

• 1.5 gallons (ferm vessel is a 3-gal carboy)
• 2 lbs light DME
• 1 oz Cascade - 30 min @ 160F
• 2/3 oz Cascade - 15 min @ 160F
• US-05 (a full packet)
• current temp is about 65F

Some possibilities I can think of for why krausen is less on this batch:

1. No boil = proteins coagulated differently than I'm used to
2. I poured the wort through a strainer instead of siphoning
3. Light & simple wort = less proteins than I'm used to?
4. Slow start = bad yeast?
5. ...though some people say US-05 can work slowly in general
6. Yeast is a living thing, thus every batch behaves differently, RDWHAHB.