I just finished up my first couple of rounds of HoN, and boy are my old DotA skills rusty. I forgot how fast the game is after playing Demigod for so long!
Overall, the OP nailed it.
To me, HoN and Demigod are two different styles of this Sub-genre, much like Counter-Strike and Quake 3 are two different styles of FPS Multiplayer. Both games share a lot in common, however they appeal to different crowds and I think this is what we're going to see. I can see HoN already capturing the masses, because DotA already did. However, a new player walking in with no DotA experience would be so overwhelmed, and by extension hated by the community, that the game itself may be self defeating. There is so little difference between the two I can't honestly see the point in buying HoN if you already play DotA besides the fresh coat of paint.
The other is, as mentioned already, is the community. I can this game community replacing DotA as the worst online community of all time. Each time a person died in both of the games I played, someone activated a vote for them to be kicked. The ganking nature of DotA is also very much alive, and Stun Locking your opponents down as opposed to clever use of your skills is a type of gameplay I don't enjoy.
In terms of what HoN does better than Demigod, I can't honestly name a thing, mostly because despite drawing inspiration from the same source they do things very differently. The lobby system is certainly better, however in terms of the actual game, I think Demigod 1-UPs it in all areas. They do have pretty water though
For me, the better game is Demigod. It's slower, more tactical nature appeals to me more than the fast nature of HoN. If you liked DotA but find Demigod not the game for you, than I believe HoN is what you've been waiting for.