I did play a lot of Dota back in the day, so I was actually pretty comfortable getting back into it. It's probably been about 5 years back been Pendragon was still a part of the Dota community. I did also play Hon to some level and a lot of Leaugue of legends too. I'll try to list the major differences.
Dota to Dota2
If I could pick the graphics of choice, the original gets my vote. The new graphics (I know its still a beta) blend everything together too much. One, there are no team colors for heroes. That pisses me off. It was especially bad my first few games when I wasn't quite sure which models were who. Two, there is no form of hero glow or anything that really differs the heroes from the creeps. Also, I'm not sure if they kept some of the delay from war3, but the backswing for heroes is very obvious. I haven't played the original Dota in a long time so I've forgotten how bad the delay was.
The new item system I dislike. The older system was better imo.
Hon to Dota2
Hon did start out as an exact copy of Dota but eventually started to introduce more unique elements. They had to do something with the release of Dota2, but I think they failed pretty hard in terms of balance.
Lol to Dota2
My number one complaint for Lol was the snowballing effect. Once behind, its very hard to pull your way through. Also, because I used to play a lot of lol recently, I found last hitting to be much harder in dota2 because everything is instant in lol; there is very little delay or backswing when you cast or attack. Having lol experience will help you depending on how skilled you were in lol. The map awareness and ganking is pretty similar.
- Sounds, some of them are odd, but I think they're enjoying to listen to.
- Graphics, I did complain, but aside from the heroes and units blending too much, its actually very clean and not distracting.
- Minimum requirements, its slightly higher than sc2. However, if you can comfortably run sc2, you can run Dota2. If you can barely run sc2, Dota2 will be a pain.
- Live watch system, I don't know if there's a limit, but you can watch live games happen. They are at least 2 minutes delayed to prevent ghosting. It's very cool if you're waiting on a friend to finish. You can watch them play. They also have a replay system too.
- Item system, just not as convenient to use, especially when the items you want aren't in the recommended items. They have some hotkey system available for the shop, but it its way to easy to accidentally buy something.
- Pause system, there's something like a 30 second wait before the other team can resume. Very annoying when the other team is full of little kiddies.
- UI, its bland, hopefully they add something to it.
In the end, its exactly what you expect, Dota on an updated engine.
(p.s I played with JP yesterday! It was sad when he told me he wasn't streaming. No screen time for me, but at least we destroyed. :|)