Jon,
  Great question.  I haven't fished many of the smaller natural lakes in the winter.  Just Sawyer and Meridan.  With very little success I might add.  Spent a good part of one day, camering in Sawyer and Meridian and didn't see anything (bass wise).  So the answer is " I don't Know ".  How about that.  I would say structure, over a specific depth.  Depth is relative to a specific lake.  Sawyer gets deep 50+ ft.  Meridan about 35ft.  Silver lake 12ft.  So it's relative to the body of water.
  When we fish Sammamish of Wash.  I look for structure.  As you know, there are places we have fished and caught fish as shallow as 34ft and as deep and 65ft.  But the fish were relating to the structure in that area.  Now most of the these smaller natural lakes don't have the deep water structure like Sam or Wash.  Smooth bowl like bottoms with soft/mucky composition.  Possible the fish in these lakes might suspend at a comfort zone and follow food sources, vs. actually relating to bottom structure and lack thereof.  Since most of my winter fishing has been Wash and Sam, this is my best guess for you.

Mark

my $.02

I've done quite a bit of winter fishing over the last three years the first of which was almost entirely lake spanaway.  These smaller lakes with smallies usually like you found jon have a silty bottom compostion and lack alot of steep drops.  what I found is that on these lakes it doesnt take much to attract these fish, perhaps due to the limit of qualitty structure at that depth.  For example one of my favorite areas on spanaway was in about 20-22ft.  This was really just a break of about two feet maybe five feet wide that was rocky for about 100ft.  yet there was usually a school of smallies on it somewhere.  I have also caught smallmouth there as shallow as 15ft in the winter.  Now on wa or sam those fish can vary like mark said from 30-70.  I think this is predominately due to structure and to a greater degree food.  for whatever reason i think most crawdad go deep this time of year and smallies know it and follow them to those deeper rock/wood.  Chris you make an excellent point about the water temp and it makes a guy wonder what lurks shallower this time of year.  However to reiterate my point I believe like mark it is mainly structure that holds craws (ie. rock,wood) that draws winter bass.  hope some of that made sense 

troy

I just had this conversation with a guide on Kentucky Lake, and we concluded that there is something more important than either structure or depth - and that is the forage.  The fact is, fish only do three things: spend one week having babies, and the rest of the time is spent eating and resting.  Since they aren't spawning in the winter that leaves the decision to go somewhere in a lake based on eating or resting.  And I don't know about you, but I can rest anywhere.  But if I am going to eat, I need to be near the frig.  

Predators like bass don't go deep in the winter to get away from the cold - it all has to do with the food chain.  There is no plankton being made in the shallows during the winter, and the smaller forage fish (shad, minnows, sticklebacks, etc) are much more sensitive and affected by water temperature changes - so they go deeper to find stable water temperatures and other sources of food.  The perch, and other panfish follow them, and the predators follow them.  

Likewise, wolves will follow caribou migrations up to 500 miles.  That doesn't stop them from eating squirrels and rabbits along the way, but they still follow their primary food source.  

All predators go where the primary forage goes.  In reservoirs where shad are the primary forage, the bass move in and out throughout the year depending on where the shad go.  Sure, being opportunists, they eat sunfish and anything else along the way, but they hang close to the primary forage.

What good is structure if there is no forage in the area?  What good is depth if the forage is not in the same depth?  

In the past two years on Lake Washington, the bass and the majority of the big perch have not been in the same places in the shallower east channel that they have been for the past 15 years.  But there is only one reason for it - something has affected the forage.  They are either deeper or suspended over deeper water.  I just haven't figured it out yet.

It doesn't matter if a lake has a maximum depth of 200 feet or 20 feet.  It doesn't matter if the lake bottom is flat and and featureless, or filled with submerged timber.  No food - no bass.  

Shallow basin lakes have always been a mystery to me because I have not taken the time to discover what is the bass' primary forage nor have I figured out where the food source goes in the winter.  You are asking good questions but the answer in my humble opinion is 1) the depth is more important if the forage is at the same depth, and 2) the structure is more important if that is where the forage are hanging out too.  But the key in either option is the forage.  Find the food, find the bass nearby.

I doubt if the craws are a primary food source in the winter (most are buried deep in the mud and won't be out until spring).

Look for anything in terms of food fish first - chances are, in a 100 acre lake, they are concentrated in 1 acre of it.  

Most people during the winter want to get on the water and start fishing right away.  Winter time fishing is all about bait fish location, and a good deal of the morning might have to be spent idling around, zig-zagging over great expanses of water until you run across some bait-fish activity.  Most anglers hate doing that.  They think it is wasting time.  So they fish where they think the fish ought to be, rather than where they are.  

Larry Gonczy and I used to spend as much as an hour int he morning on Lake Washington watching the electronics and idling around searching for some indication of life.  

In the winter time, your electronics definitely become your eyes.  Find some perch or stickleback activity and the bass will be close by.  Now the only challenge is to catch them.  (You can see why the majority of anglers winterize their boats and leave them in the garage until spring.)  But when the conditions are just right it can be a great day on the water!