If ST were going to revisit this, I think a clear framework for evaluating options might be a good start - especially if fleshing out the GUI to let users control them isn't practical. Right up the top I think productivity would be a key. For brevity I won't expand on that but I don't think consensus would be difficult to find. In evaluating where a GUI has the potential to help increase productivity every bit of screen real estate counts, even well placed white space.
Every column and every field should serve a purpose - what do we gain by including this vs. what do we loose (or gain or make space for) by not including it? Does a 3rd line just to display the size of the message offer anything of value for the space it takes up? (just as an example).
Then, and this is important too, user expectations. Is it intuitive and easy to work in or will the need to train or explain along with frequent support tickets be the norm? Does it follow industry standards/norms to help ensure it seems familiar and intuitive; or, if we deviate or introduce something new, does it truly bring something to the table?
Finally, has every update that would have a user do something differently, or see an altered default layout, been carefully considered? Does the change become an Easter egg hunt or is there some sort of alert if things aren't obvious. One of the larger complaints we hear about Microsoft's suite of products now are frivolous changes to the user interface where a user arrives in the morning, jumps in, in a hurry to catch up on a bunch of things, and familiar tools have moved or the user defaults have been abandoned or changed.
At the end of the day, I think it's about productivity and equally about user expectations - users seldom appreciate change unless it brings something to the table. Many oppose something going missing even if they didn't use it. If they do come to the web interface, they are probably coming with familiarity with some other interface and, if there are norms, we might want to take them into consideration. If we want to drag users from Outlook to make additional features available to them in the Web interface then an Outlook sort of look, although not essential, wouldn't be a bad start. Using common Outlook configurations as a guide to the type of information that should be displayed might be a good start. An example of that might be the way (and for which days) Outlook displays the time of messages as well as the date. Important? - I honestly don't know, I don't use it, but many Outlook users would expect to see it there and the big drive in Enterprise has been to accommodate and attract just those users. That group we've heard from ST is a minority but, at the same time, critical in the Enterprise market.