The M2 is interesting because at Tier 3, it's mostly fighting even lighter vehicles; Tier 2 and Tier 3 lights. Even in matches against heavier armor, it's still encountering a large proportion of lighter tanks. That has two effects.
One, it fights a lot of things so lightly armored that the 75mm HE shell will blow right through its armor and do full damage. That doesn't happen as often with the later 105mm howitzers, though it's a common experience for the KV2's 152mm derp gun.
Two, relative to other units in its generation, you have the HP and armor to play 'battlefield bully' a bit. Very few Tier 2 or 3 tanks will still be there if you shoot them twice. You can do a lot more damage with those two shots than they can by banging away with 20mm or 37mm guns for the five seconds or so it takes you to reload. So when I was in the M2, I never feared the outcome of a close encounter with a single enemy, unless they were agile enough to literally drive rings around me- I hadn't got the hang of hull+turret turning and my graphics made it hard to fight under those conditions.
I had a lot of success in the M2 Medium, both as a support tank like Zinegata says to try, and as a brawler. For a long time it was the only tank I'd won a Top Gun in, during the period when my laptop drivers were crappy, my game had 100ms lag at almost all times, and it kept crashing on me at least once per match so I had to reboot while my tank just sat there getting shot at.
That may have been luck, but any tank that even I could score seven kills in under those conditions can't be all bad...
