In-progress-ness nonwithstanding, the differences are.... the paint job. And the UI a bit I suppose. It's a mechanic-for-mechanic translation, mainly because Dota fans are fiddly enough to revolt if they didn't.
That's actually a complete misrepresentation of what the transition to the Valve engine adds.
The people who think that a game with the popularity and money making potential DotA had was best suited to stay on the dated and badly supported Warcraft 3 engine, have no idea what they're talking about.
The move to the Source Engine added:
1. Reconnect Feature.
2. Spectator Mode for hundreds of players (planned to be limitless).
3. Tons of new spectator features such as auto-camera.
4. MUCH better netcoding/massive reduction of lag and input delay.
5. Built-in bot support.
6. Ability to make new maps for the same game.
7. Leaver protection.
8. Team matchmaking.
9. Stat tracking.
10. Better chat/friend management.
11. Hacking protection.
12. Much superior graphics/visuals.
13. Built on the Steam engine.
14. Completely free (no need to buy Warcraft 3 first).
15. Community-built skins and items.
- Many more, too many to list here.
The game mechanics didn't need to change, because the mechanics were fine; they've been developed over 10 years. They really needed an engine switch and an updated interface. The people who don't understand this have no business sense and/or never played on the Warcraft 3 engine 10 years after it was made.