The major problem with making an MMORPG with a highly influencing storyline, similar to a single-player RPG's, is that MMORPGs are anachronistic in nature. Unless you make extensive usage of a phasing system like World of Warcraft now has (where areas change as you complete certain quests or fulfill certain requirements), you can't alter the game world majorly when one person does something, or it prevents every other player from doing the same thing. This might work for once in a lifetime events, but it also cuts off other players from experiencing content.
Some MMOs try to do this in other ways, like EVE and its system of sovereignty where players can take over large sectors of space. This works to some extent, but in this case the players are writing the story more than the developers are, and this can be a barrier to entry for some players.
The major problem with developing an MMO like a single-player RPG would be how much things have to be rewritten when a major event happens. Some quests in WoW still reference Onyxia as being a looming threat, despite the fact that canonically she's been dead for a very long time. Now, Blizzard has done a generally good job of remembering some of the shortcomings of, say, turning Onyxia into Lv80 content for WoW's fifth anniversary, including moving her old tier drop (T2 helmets) to Nefarian, but it's a lot of extra work when you have to rewrite a few hundred or so quests, or create entirely new ones, because one big baddie is wormfood.
tl;dr, Well, I wish Bioware the best with breaking the MMORPG lack of time passage. Hopefully the game doesn't wind up being frustrating for new players because they can't play with their friends.