Ian makes great points. I would summarize it as too many generations of LW gear sitting on the shelves at home, but not enough in your pack when you need it.
I also learned modern LW backpacking at this site, and I also went too light, in 3 ways:
Safety is compromised. Numerous examples were related above. To throw in my worst case, I had to extract a severely ill companion on a 3,000m+ alpine route- (Miguel and Michael, you know it as the daikiretsu to Yarigatake..)the only viable escape route would have been relatively easy with crampons one axe, and about 20m of rope, but we did not have these because the planned route did not require them and I was on a fast&light summit attempt. We tied every piece of webbing, cord, lanyard, etc. together and I used body bollard, and boot belays to lower my companion. It was ugly. This year on summer routes above 3,000m I will carry 20m of 6mm, Grivel Spider crampons(140g), and a 50cm CampXLA210.
The second example of going too light is when comfort is compromised too much. For example having a too-cold sleeping system or a painful 'onion-sack' of a backpack. I think of spectrum of comfort with 'traveling comfort = light' at one end, and 'in-camp comfort = heavy' at the other end. I make choices from my gear inventory based on the ratio of traveling vs. in-camp time.
Lastly, economy is compromised too much. This has two components,
a. gear which is replaced with even lighter gear: For most requirements I have a succession of lighter and newer gear to fulfill it. As a result, most of it never gets used. That was just a waste of money. The solution is to learn from sites like BPL, but reduce the frequency of purchases.
b.gear which wears out too quickly. Examples are paper-thin rain wear and a light backpack which both tore first day of use.
I have now settled on a core group of gear which is light but also durable and versatile.