Framebuilders - Compromises in a do it all bike?

I'm thinking of what I'll call, for lack of better word, a do it all (or mostly) bike. I need some help understanding what compromises in design or whatever that I'll have to make. The bike would be steel and lugged. I'm in fairly fit shape and an experienced rider (mountain and road) and weigh 155 lbs. I would like a bike that can handle fast road rides, dirt fire road epics, maybe some single track and touring. Admittedly, 99% of the time will probably be the road and dirt riding, but I'd like the ability to mount racks and go on a touring trip. I'm not looking for any kind of suspension. I'll probably enter an occasional cross-race, but I'm mostly just an aggressive weekend warrior.

What are some of the compromises that I'll have to make? For example, since I want to use the bike as a touring bike does this mean that thicker tubes should be used for strength at the expense of weight? My road bike is a custom Serotta and I love the way it fits and handles, but there isn't much clearance for fat tires. I guess I'm just wondering if there's a way to have my cake and eat it too.

You can have your cake and eat it but the compromises of a medium, club style tourer mean that it is a Jack of all Cakes.
US tourers tend to be heavier duty than UK versions. The typical UK club touring bike rides well unladed and is good for a few weeks of loaded touring but is not as good for prolonged continental expeditions on rough roads.
Check out Bob Jackson, Mercian an M Steel for some inspiration.
Try and find a copy of Touring Bikes by Tony Oliver. He discusses the compromises neccessary to make a well balanced touring bike for various roles.

cross-check surly

my poprad if it had a tad stiffer rear triangle (for loaded rack duty)

The biggest compromise there is the touring part. If you want to use panniers with real heel clearance, you need long stays. Long stays aren't all that compatible with everything else you want to do with the bike.

Personally I think a "29er cyclocross" style frame would be the way to go. Essentially a 29er that only has room for say 35c tires max. I'd get two forks for it too - one rigid one suspension - and probably consider S&S Couplers too if you travel much.

Titanium would be the ultimate material but powdercoated steel would also work well.

My all around bike. Touring, offroad, cruising, and you name it.

How would longer chain stays affect the ride? Would the bike just flex a little more under a sprint? Also, why would you limit tire clearance to just a 35mm? What's lost by going with more clearance? I like the idea of a 29"/cross bike - but since I wouldn't have suspension I was going to look to a fat tire to handle the cush - something like a 2.35 (which I think is bigger than a 35mm). Ideally, I could easily swap between two sets of handlebars - a drop bar and a straight bar. Are there such things as "quick disconnects" for cables, i.e., some type of coupler system that allows you to easily take apart a cable?

Yes, Bruce Gordon amongst others makes splittable cable connectors. I personally like the idea of running full length cables on my travel bikes so I don't use them because you don't need them.

The reason I suggest 35mm tires as a max is so it's more versatile. If you design a bike to potentially run 2.3" (58mm), that's fine when you're mountain biking but putting road tires on there looks a little strange. No big deal though - if you need to run 2.3's, design around them, but make sure the bike still works with road going tires.

Two sets of bars is not a bad idea, but off-road dirt drops might be a good compromise.

There are many possibilities. Here's an S&S touring/singlespeed/do-it-all frame we're doing for a customer right Now. (http://www.thylacinecycles.com/gallery/press/Thy_BC_AreteS_S01_A3)