I have been tempted as a DM to invoke time limits to tactical discussions. On occasion I jokingly threaten to invoke a mythical "real time is game time" rule. But I never do. Usually the joking gets things moving well enough.
Since I play and DM in about equal measure, I see both sides of the debate on this pretty clearly. As a DM I'm trying to pace the game and keep people interested and engaged. As a player I'm trying to make the best possible decision under the circumstances. These sometimes are competing priorities.
In the main group I DM the most for, we have a classic "casual gamer" player. He is never prepared (he usually forgets his dice and has even been known to forget his character sheet), he never bothers to learn his character, he constantly asks what he should be doing, and a significant amount of the time, he simply allows the group to control his character by default. We've been playing with him for several years, he's not going to change. He plays to get out of the house and spend some time with the guys. That's about it. So since he's a good friend we don't have a problem with it, although I do wish he would spend a little time learning his character and making his own decisions. I've got a lot more important things to worry about though.
I'm fairly new to 4e, I am playing my sixth session with my first 4e character tomorrow. Twice so far another player has provided guidance to me on how to better utilize my powers and skills. I was a little embarrassed to have made the mistakes I made (in one case I did not take an optimal path and so ran, he showed me a way to make the jump across the open space and reach the enemy without running, in another he reminded me that I could take a move action as a minor action and still apply my hunter's quarry to a new target). In spite of the embarrassment though, I politely thanked him and followed his advice. In both cases I could have figured it out on my own, but playing with a new group I was feeling a little pressure and wasn't thinking as clearly as I should have. I'm glad he helped because that is what my character would have done and it was better for the group.
When our group moves into battle, we have a lot of tactical crosstalk. A good chunk of it is clearly meta-gaming. Our cleric works out buffs or heals and tells another player when on the battlefield, they might be 50 feet from each other. I do feel this has a negative impact on encounter durations and I would prefer we did not have as much meta-gaming crosstalk, but I don't find that it is anything that I would raise as an issue. That's how this group works, I joined them after they had already developed habits.
In terms of "how much of a problem is it for the game?" my answer is not much at all. In general it's a minor annoyance at worst. For many situations the time lost in this sort of crosstalk is less than when a player loses a die under the table. In terms of actual game play impact and annoyance, in pretty much every game I play, players leaving the table to get a drink, hit the restroom, talk on their phone, talk to a family member, etc. are all much more disruptive to the game and irritating to the other players.
