Best arts/moba game out there? sure! Once you try it you can't stop playing it.
So, how should I start? I obviously won't tell you anything about it's lore, heroes and such, as you need to try W3 first to understand what's going on and how you need to play it. If you didn't, please, do it now so you won't get flamed.
Edit : I forgot to mention that W3 is a little bit hardcore, it's harder than D2 in every aspect. There you won't get instructions on what items should you buy or what skills should you level. . I learned DotA the hard way, not from these guides that you have ingam, inventory shorcuts and stuff like that.
So, source engine brings more features, better graphics and gameplay to the old thing; that and a shop with cosmetic items that your heroes can wear in battle. Since it's free, valve, icefrog and even you can get income. The workshop allows you to craft your own items and couriers and get cash and fame :D.
Now, the gameplay has been improved a lot. When your hero is on low hp different animations and poses show up and you'll struggle to run while beign injured and dying. Heroes can say different things to the enemies or allies on your lane when you kill them, steal creeps or denying them. When using things like 'ty, lol, haha, jaja' in chat your hero will say something, or laugh. Small things like that adds greatly to the immersion and it's cool to witness.
The sounds and music are just right, nothing to say about that atm. The graphics too. The thing is that updates will always come with updated textures and new stuff.
And the game is rewarding as hell, not only ingame items and battlepoints, cosmetics, but mental too. That only if you know at least the basics. Good thing now that valve added a couple of quests that will prepare you(almost) for the many hours of pleasure and pain that will come.
Oh, and why 9.5? The network keeps crashing lately after so many players; and the game too. These are the only negative features that I can recall.