One huge pet peeve is the behaviour of the automatic transmission which always downshifts to first and makes you slow down much more than needed. I just checked SpinTires new beta and there the automatic transmission downshifts properly allowing you to keep as much momentum as possible.
Otherwise I second the weird traction on pavement. I think they just made it so that weaker trucks can't pull big loads uphill but they should have just made the engines stall instead (yes, even in automatic and first gear - so that you would need to get a running start before reaching the hill).
I would also like to see maybe a small slider to control the points needed for each lumber mill, on choosing a map screen. Something like 8-12-16 would be nice. That should also allow the community to make interesting maps without having to add 5+ lumber mills to compensate.
From what I'm seeing MR2 will feature quite a bit of a gameplay change so it would be nice if we could still go back to MR1 for the 'old-school' style. Of course, we could only do that if it were properly finished first.