1) Multiplayer in general - Finding and connecting games via the ingame system is horrid. We have been using GameRanger to get solid quality games going since release almost.
2) Parth + AI - You CANNOT HAVE AI PLAYERS IN PARTH MATCHES. AI players are stupid and feed gold/xp. That is just about all they are good for. Either the AI needs to be vastly improved somehow. OR they need to "cheat" and by cheat I mean. If you kill them they vanish and wait for respawn but provide no gold or xp. Basically they are a super buff creep assisting you.
The fix? Adopt WC3 Ladder play rules. WC3 is perfect and has been doing great. Just friggin copy their ladder rules. Game must last several minutes before it counts as a win. If you LEAVE the game you get a loss (if networking issues and random disconnects continue to be a huge problem then ladder matches are a joke anyway). Have several game modes to choose from. Those being:
a) Parthanon Standard - You are randomly put into a match with random players and arranged on teams based on rank (Highest 2 ranked each are split... next highest 2 are split... next highest 2 are split... basically even teams).
Parthanon Arranged Team - You can play with friends! However this cannot be the same ladder as people who randomly join vs other random players. Its not fair to have an arranged team of 2-5 players on voice chat with a perfected strat and selected heros to fight against any random group of "noobs"
3) Camera - It is completely stupid that when you die the game spins the camera around and places you facing some random directions. Do not change the view angle for the player. Let them do it and leave it alone!
4) Effect immunities - Stuns - If you stack a team with stuns you can perma-stun players to death with little trouble. I don't believe there are any items you can get to counter this. Dota uses an item that gives temp immunity to all effects (almost all). And also has an item which blocks spells cast on you every several seconds. No matter how its fixed something needs done. A team with 2+ vamps can warp in and chain stun someone for 6+ seconds and do massive damage... there is no way to counter this unless you have oak.
5) Better tool tips - There are several abilities in the game which have magical bonuses that aren't listed on the tooltips. I can start a list below eventually. However every "effect" on an ability and everything it does should be listed for the player.