It has an epic storyline and incredible online features.
The gameplay is vastly different to many other real- time strategy games, because some units have special abilities and magic which can do any number of things, from healing your units to summoning wolves, to even ressurecting dead troops to fight for you (regardless of whom they belonged to before). Obviously there are tight restrictions on how often the more powerful abilities can be used, and so they should be used tacticly.
Some people have made custom maps that only use the basic game engine to make all sorts of other ways to play online, such as Line Tower Wars, DotA (VERY popular), Footman Frenzy, Wintermaul and Uther Party (Tons of minigames, I think there were about 50 in the last version I played).
There is also an expansion pack, called The Frozen Throne, which improves even further on the gameplay.
Friend: Standard game then? Or do you want to do some DotA?
Me: Actally I feel like playing Line Tower Wars.