M9 is based off of M8, so if you play M8 then you're going to have an easy transition. The biggest change is that each team can have 1 of any hero, regardless of what the other team selects. Rine was too powerful with his lv3 in M8, so it fires 1 less shot now. For too long spawn rushing = win in tournaments, so it's gone. Another new features is Enemy spawn increasing. Your team can spend 3 civs to make ur enemy's warp gates pump out and extra spawn each wave. It should pay for itself after about 8 minutes. Medic has a new lv1 by default that makes it hard for noobs to feed with her, but you can have the classic lv1 if you suicide your infested terran at the top in the first 30 seconds. The new 1 spawns a medic at each of your allies to heal around 90 hp, then removes itself. Double casting increases the time and heals the healing spirit. Her Lv4 now heals +3 levels of HP more than it used to. Summoner now can get his goblin speed and attack speed with his 2nd and 3rd spell level. Lv4 has also been boosted. Abyssal demons(max 4) are immune to Warrior lv3, and the can only die 1 at a time by other kill effects. Mech's lv2 rail gun only fires while mech is moving, but it doesn't block your path. The mutant lv3, and assasin lv1 no longer channel. LM's lv3 does not take as long to go off now. Assault's lv1 now does a slow drain, and lv4 no longer has a pre-spell effect (so you can be sneaky). Gates take 100s longer to respawn. No more invincibilty at temple, you heal every 7 seconds. This means summoner is not safe at temple against a dt late game.
If you play 3.4 ~ Well I made 3.4, but this is definitely not 3.4. Just trust that if I was able to balance 3.4, I can balance this.

M9b fixes bomber glitch, M9c fixes volt lv1 glitch, M9d fixes hallu glitches + assault lv4 + path-blocking doodad and some name corrections.
M9e truly fixed DM lv4, Unrigged E. spawn upping, 7 second cooldown on heal only happens if your base only commands 1 warp gate spawner.
M9T fixes mech lv3 death, assassin has a base of 44, and special thanks replaced with everyone that signed up for the tourny (on the website).