My first impression is very simple: I want my (polearm) quick melee back.
Milling through crowds at full tilt is what has made Warframe enjoyable for me for the past few years, and that appears to have been removed.
I'm going to poke around and see if this is a configurable thing, but right now I'm just... very disappointed.
Edit: So, i've thought about it a bit, and I think my fundamental issue is an aversion to anything that interrupts my controlled forward motion - such things make me frustrated on good days, and motion sick on bad ones. I'm here to be a Space Ninja, after all, and that's about moving and killing. I can get back to doing this with the new system if I can get one thing: a stance that has a primary combo that does not have movement-interrupting footwork. If I can just keep moving forward at full tilt while laying about me with a melee weapon, I won't quibble about whether it's a combo, a weapon in hand, quick melee, whatever.
Right now, though, I still feel like someone has pulled one of the spark plugs out of my car's engine - I hit the gas, and get nothing but hesitation.
Edit 2: further expansion.
Problem: as a consequence of the melee changes, players who relied on or were accustomed to the smooth, high speed quick melee behavior of certain stances (primarily Shimmering Blight and Bleeding Willow, but also the nunchuck stance and possibly others) are finding themselves frustrated by interrupted movement and unwanted attack animations.
The melee stances created by the Warframe team are spectacular examples of choreography, animation, and (I presume) motion capture. By and large, however, they rely extensively on animation locking and stationary attacks that are not a good match for the high speed, high mobility, fluid, parkour movement style that is core to the general Warframe experience.
Solution: without discarding the new melee system or negatively impacting the existing stances (or dismissing the considerable sunk costs to make them), I believe the best path forward is to add a line of new, simple stances that capture the old quick melee experience for players who seek it. These needn't have more than the bare minimum combos required for normal functionality; players who want this simply aren't interested in the complex timing and multiple inputs of the high-impact stance combos (with their damage multipliers, slam attacks, and the like). With luck, these basic stances can be cobbled together from existing assets. Done well, this will have the positive side effect of increasing the number of weapon classes that players will employ (ie, we won't feel restricted to only polearms, simply due to movement considerations).
Resolving this player pain point is something that I am entirely confident that DE can do, and do well.