Since there aren't really, well, any unarmed combat rules whatsoever, isn't saying "You can trip a lot with martial arts" sort've pointless? There's no martial arts in the ruleset. Yet.
Yep. I've been trying to figure out how to say, "Isn't all of this just a fancy way of saying that the rules don't support the Monk yet?" But I couldn't figure out how to do it.
Unless people really are defending the spiked chain machine gun tripper. I'm pretty sure they aren't though.
At the moment, I can create an Iron Vanguard Fighter, who can push people at will, push them in fancy ways a few times per encounter, trip with two encounter abilities (technically could be three, but I'm assuming the level 1 trip gets replaced with a better one at higher levels), and trip with two daily abilities. This is a viable (even optimized) character. He knocks his foes around like pinballs.
So... lets try to keep this grounded in reality. 4e lets you trip your opponents and knock them about pretty frequently, even including only the core rules powers. This is not a debate between "once per fight" and "whenever my style justifies it," this is a debate between "a couple of times per fight, plus you get some other stuff to round out your style," and "over and over and over and over because 'my style' really means my only combo attack."
For a guy in heavy armor fighting with a weapon, what we have seems a plausible amount of tripping.
And honestly, there's nothing wrong with describing the last attack versus an enemy as knocking him to the floor and finishing him off. I was doing that in 3e, for crying out loud. I can do it in 4.