Well considering the OP's dichotomy of action vs. RPG (which I take to mean combat decided by numbers and dice rolls), I took it to mean in reference to its combat. If it was speaking to the increased focus on combat in 2 and 3, that doesn't really tell us what is more or less an RPG, given games like Wizardry that are almost entirely about combat yet are certainly RPGs.
There is a dichotomy between action and RPG. Action games tend to rely on the player's skills rather than the character's. That said, ME always was marketed as an action RPG, though ME1 included weapon specific skills that increased the characters' (including squadmates) effectiveness with specific weapons. It also included non-combat skills.
I did not find any of the exploration in ME1 provided role-playing opportunities. Unless you mean a Shepard who knew the act of exploring was stupid given the plot.
I take it you're not interested in creating emergent narrative via gameplay, then?
I don't see what corridors have to do with RPG vs. action, but I don't find ME1's arenas to be meaningfully different in this sense.
When an enemy stronghold is sitting on the middle of a planet's map, Shepard has multiple approach options. You can rush it with the Mako (from whatever direction you choose), park the Mako and snipe, etc. There are more opportunities to employ different tactics, and express what kind of strategist Shepard is.
The more cinematic dialogue and fewer middle options are indeed minuses against it, though I tend to think the Reputation system balances that out by not punishing players for playing something other than pure Paragon or Renegade.
That was certainly welcome after the P/R mechanics of ME2.