The Book of Lairs series had its moments. I think the big issue with them was that they did not follow through on the concept as much as I think they could have.
The maps and art for same were pretty boring and uninteresting, most of the encounters failed to develop the tactics as well as they could have or lend real detail and inspiration, and in general the idea seemed not to be as well executed in practice as it was when originally pitched I suspect.
How to make it better?
Better art. Better maps. Better encounter tactics and development descriptions. Significant thought given to spellcasting development. Player maps without DM notations as an online product. Better and unique treasure. Better indexing so that we can not only find what we want from the product - but be able to pick and choose via level and habitat so that we can locate and lift encounter elements from it more easly as we need to.
I'd buy this product - and happily so - if WotC followed through on the concept and didn't go through the motions. It's tempting to see this as an easy to do product that you can throw inexpereienced designers at and just churn out crap for people to buy.
If WotC takes this approach - they should not be surprised when customers correctly conclude that the product is worth what WotC put into it (which is to say - not much).
But if they want to take the time to develop it properly and put the effort in - I think it's a great generic non-setting specific way to sell product, especially attractive to those who home brew.
As individual "Critical Threats", they are also easy to publish on the Web.