Hey, Sunstorm -- I think, in part, you've largely answered your own question: Your problem with your current ride has less to do with weight, and lots to do with geometry. I can stand on my bike whether its loaded or not, and my ability to balance isn't affected. If you can't stand, then there's something fundamentally wrong with how you and the bike fit together. For my money (which it isn't) this is an excellent reason to shop for a new bike! And, I think, by definition, if the folks at the Trek shop will fit you appropriately, then you should be able to get a fine ride there. Which ride you ultimately choose is far more personal.

Ah, the variety of the human condition... Hanging around here, I think you'll ultimately come to the conclusion that there is no perfect commuter... just the best commuter for each individual. We all have different constraints, based on riding style, load carrying needs, comfort, etc... Take handle bar choices: Some like flat bar asphalt bikes. I prefer drop bars. Others like trekking bars or moustache bars. And that's only one component. So you need to decide your preferences, and then find the right bike within your own preference. As long as it fits, you'll likely be a happy cyclist for some time to come.

I ride a rigid hybrid with drop bars as my daily commuter, but I also have a front suspension MTB that I use as a backup. While I don't find it as comfortable for longer rides (>15 miles), I do find it a perfectly acceptable commuter up to 12 miles each way or so. I should mention that's on knobby tires, no bar ends or other ergonomic improvements, etc... With a little love, I could probably turn the MTB into a great daily commuter. Others love their MTBs for commuting, and ride them regularly, in any number of variations.

At this point, the conversation turns more to what makes you comfortable while you ride, and what features you want. Things like disc brakes are nice -- I have them on my hybrid -- but they aren't really ~necessary~ and you might be better served to choose the next bike up the line with standard V brakes, to get the other improvements in components and possibly weight savings. OTOH, if you know you'll be riding in all weather, with frequent rain and possibly sleet or such (cause it doesn't really snow too often in Cary), the discs might be well worth it.

I have to admit, I would not have expected a 'modern' aluminum frame bike to weigh upwards of 40 lbs naked. Ouch! I still don't think the weight is the major part of your problem, but it probably isn't helping, either.