5-box leveling, balancing efficiency and "safety" ?

[Apatheist]

I'm still torn between leveling a trinity team from the get-go, or a tank/healer duo and then PLing the rest after I hit 60. I wonder if replacing a mage with a balance druid for the crit aura would be worth it, especially if running a pally tank?

If you just want a level 60 druid, swapping a mage out won't prevent you from clearing dungeons. Any tank, healer and 3*DPS combo will be able to clear dungeons. Some comps will take longer and may not be as viable in PvP. If you're doing it in order to gain DPS though, you'll lose more DPS replacing a mage with an oomkin than 3% crit will give you. A warlock would increase party DPS and provide more utility.

As far as efficiency goes -- I'm fairly sure you'll get 5 characters to 60 faster by grinding dungeons together rather than leveling in two separate groups. If you level in dungeons from the start you'll be in pretty much full blues by the time you hit 60 and geared enough to start farming preraid BiS. Here's a preraid BiS list so you can get an idea which pieces to aim for while leveling. As I've said before, the stat difference between rare and epic gear is immense in classic -- much more so than in other expansions. There are some level 40-50 rares and epics that are better than raid tier gear. Warden Staff and Smoking Heart of the Mountain are good examples. Warden Staff is a level 43 item but is better than Naxx tier weapons for a feral tank.

I think they can get away with a lot of that without impacting the "flavor" of classic (e.g. quest progression rates, dungeons, character power, talents, etc.).

I'm one of those vehement "no changes" people but I can't see an issue with them improving back-end stuff, changing the launcher, etc. As long as the gameplay experience remains unchanged I'm fine. The fact that you can trade loot already put me on tilt a little. It shows they're willing to rationalize changes to actual game mechanics which (IMO) probably means we'll see them messing with PvP /follow and other nonsense that doesn't belong in classic.