I don't envy anyone who must attempt a remake of a classic film. And especially with sci-fi, because it's too easy to run afoul of obsessive types who have probably memorized whole passages of the film, analyzed it shot-by-shot to look for production gaffes, and have probably fashioned their own robots and space suits in its honor. (And if there is not already a Gort flashlight tie-in, rest assured that industrious fans are probably already on that too.) So you're basically setting yourself up for failure by even trying. If its too much like the original then it "didn't bring anything new to the story", and if you did bring something new to the story then you "butchered a classic."
This isn't to completely let the recent Keanu Reeves blank-stare-a-thon that is The Day The Earth Stood Still off the hook, but more to keep everyone's expectations in check. This is a big-budget remake of a movie about (a) a great big robot named Gort capable of inflicting some damage, and (b) a humanesque ambassador sent to deliver the bad news that (c) humans are busy killing themselves, and the aliens will have none of that. So let's not get too precious about our refined sci-fi plot elements.
In fact, a couple of the main criticisms of this film are, on closer inspection, actually very much in keeping with the original. Yes, on this movie they did get all earth-friendly, making the danger that we're inflicting on ourselves an ecological one that's damaging our planet. But in fairness, it is far more subtle than the original's admonition of "No Nukes!" As the screenwriter points out on the commentary track, The Day The Earth Stood Still is very much a message film. That was the whole plot and point of the original, and it's silly to expect that you'd have the same story without it. Granted the subtlety is managed with far less grace than the original - some genuinely silly lines in this update shine brighter than Gort's laser eye - but it's a necessary element that carried over reasonably well, all other alternatives considered.
Overall, the acting and the visual effects are well done. Although Keanu Reeves isn't exactly a national acting treasure, I will grant that he can play a fairly effective robotic alien. Jennifer Connelly is quite good as the scientist/mother who is trying to convince Klaatu that we're worth saving, and Jaden Smith as her stepson is appropriately bratty. Although the pacing is often rather slow for an action movie, this also is very much a carryover from the original, which was primarily a human drama that just happened to have a sci-fi underlying story. The update naturally turns up the special effects knobs a few turns, but it still keeps this core of a drama.