Pure speculation, and of course everything is fluid, but interesting to ponder what the others party strategists might think of a Dion-led Liberal Party:
Rumor has it that Dion was the preferred candidate of the big three. The Conservatives might see an opening in Ontario, because Dion didn't score particularly well there in the delegate selection. I also think the Conservatives could argue that Dion won't play well in rural Ontario.
The natural assumption that Dion could be rejected in the West. Alberta was always a longshot for anyone, but I think the Conservatives might see an advantage in the "another Quebecer" angle from Manitoba westward. I would think there would be some hesitation in British Columbia, where Dion's policies might find support.
On the surface, Harper might see opportunity in Quebec and will use Dion's past to hurt his chances. However, any enthusiasm should be tempered by the native son angle, which could translate into potential problems. On the issues Dion is far closer to Quebecers than Harper, but compared to Ignatieff or Rae the Conservatives are probably happy.
The NDP probably wished for Ignatieff instead of Dion. Now the landscape has another environmental player, which further erodes Layton's arguments for relevance. Dion's policies should play well in the Toronto area and the lower mainland in British Columbia, which are essential areas for the NDP.
Dion moves the Liberals left slightly, which means the NDP has less room to make the distinction. Layton also loses his trusted line of the "arrogant, corrupt Liberals" because he is on record speaking about Dion's integrity. Layton is probably quite worried about Dion, although this will be tempered because he might be seen as better than Rae.
You would have to think the Bloc is quite happy with the Dion choice. Ignatieff and Rae were both problematic, but Dion has baggage. Duceppe probably relishes the opportunity to make the election in Quebec a pointed discussion about federalism. Lots of talking points for the Bloc, plenty of opportunity to try and trip up Dion. The only worries for the Bloc, Dion knows how to fight back and could be a force in any debates. The Bloc might also worry about Dion's ability to re-invent himself with Quebec, given the fact a francophone may enjoy some openness.
May likes to argue that she is in favor of anyone who moves the environmental agenda forward. Dion should make May happy, but I don't think she should be if seat gains are her goal. There is a danger that Dion could take the environmental vote and make the Green momentum a past tense. May will probably win a seat, but I doubt the Greens welcome the prospect of a crowded environmental field, where they are decided underdog. Dion is probably the worst choice from the Green perspective.
disclaimer-I didn't address how Dion can overcome any apparent shortcomings. I'm not claiming the perceptions are correct or long lasting, just relative preferences.