I'm going to break this down into the most important points that highlight the differences between Blizzards Moba and the competition.I will only used Dota, as I have never played LoL.
I want to use short and sweet points.
HOTS is easier than DOTA.
HOTS has a lower personal skill ceiling than Dota.
HOTS has no Items, only talents you get every 3/4 levels.
HOTS has a Group leveling based System, not Solo.
HOTS has 7 maps, not just 1.
HOTS Maps are all objective based on top of the usual, destroy the Core.
HOTS has half, if not lower average game time of DOTA -20 minutes.
HOTS has NO ALL chat, you cannot communicate with the opposite team at all.
HOTS you have to buy the hero's, either with real money or gold earned in game(You can acquire 1 hero a week with about 3 hours a day of play give or take no money spent.).
HOTS Early game is VERY forgiving, almost to the point of inconsequential.
HOTS has fantastic comeback mechanics, the game is not over till its over, ever. Meaning you are never locked into a game that was lost in the first 10 minutes like in dota.
HOTS is much more fast paced than Dota.
HOTS has a little bit less depth than Dota.
All in all I think HOTS is a great game, I think Dota has more depth but also has a much higher barrier of entry, HOTS is a very casual friendly game, and that fact coupled with the 20 min average game time, is exactly why I prefer HOTS over Dota. Dota Of course is still a great game.