To me, Hawken feels much more mech than arena shooter. Although there's an experience/relativity component inherent in these types of conversations, I see Hawken as having a number of "mech properties" asides from turn-rate cap. They have two independently firing weapons, for example. There are different mech classes with different armour levels, movement capabilities and speeds - even within an overarching grouping (A's, B's, C's). The mechs experience acceleration, whereby directional shifts while walking take time (although not so much time that it turns cumbersome, a la Ascension). The mechs can boost forward to travel more quickly. They can thrust themselves through the air (and contrary to some players, I feel as though the movement speeds through the air are quite slow and "mech-like"). And even now, the mechs have a considerably higher TTK than would be present in arena shooters. Simply put, I feel as though I'm piloting a relatively fast mech.
This is what I would consider an arena shooter (twitch-based):
To me, there are massive differences between that sort of gameplay and Hawken - even if both games possess more movement than has typically been seen within the shooter genre (which I think is a great thing).
I think that the turn-rate cap is an important component of Hawken, and I think it interacts with the aforementioned elements to produce a faster-paced mech game. And while people certainly will have their notions about what is/isn't a mech game, I think that Hawken does a good job matching the game's original "vision".
Fair enough.