Here's my take on it:
(1) The only new thing is the option to let the game choose conversation options for you. As a rule, I don't mind that, but there are two problems with this:
(1a) Decisions that affect the story - such as killing or sparing someone, destroying or keeping some information etc.. - should not be made automatically. The same with decisions that define key characteristics of your Shepard. For instance, take the first conversation with TIM in ME2. The only real choice in there is to define how you stand with TIM. If you wanted to go through that fast, it would be feasible to just make one choice defining if you're his enemy, his friend or reserve judgment, and let the rest of the conversation play out automatically, skipping the part where you ask about individual old team members altogether. That would be OK for "Action Mode". But that one choice must be there.
The thing is, everything else but those decisions actually *is* just for flair and fleshing your Shepard out - "pure" roleplaying, in other words. It's been that way in ME1 and ME2, too - you have long conversations with some people, but in the end, it all comes down to a choice between two or three options. If in "Action Mode", these choices remain available and everything else is chosen automatically, you have more of an interactive movie with a more defined protagonist than a roleplaying game, but it won't necessarily lessen the experience.
(1b) I have issues with the kind of options the game will choose for you. Inevitably, they will come to shape some sort of "canonical" Shepard. I consider it important that these are suitably neutral.
(2) I don't think these new modes will harm ME3 in any way. I'm not so sure about future games, however. I'm afraid of a mechanism like this:
*The presence of an action mode will pull shooter players into ME3
*At some time, a majority of ME3 players will be shooter players.
*Someone discovers that only 30% of all players use story or rpg mode
*In the next game, there will be only action mode.
For ME3, my opinion about the new modes will rest on the answer to the question: in Action Mode, will story-affecting decisions be made automatically, too? If yes, this is very, very bad, and on top of that, I'll probably have issues with the choices the game makes for the players. if no, it's an interesting new option I'll probably never use, with some potential to ruin future games, but I won't mind its presence in ME3.