Many of the spells in 5th edition are now using concentration, a new concept. If you want to fly, you can cast the Fly spell, but you will need to concentrate on it. Not only does that mean that if you are hit by an arrow, you have to do a concentration check or fall to the ground; it also means that you can only concentrate on one spell at once. Flying, yes, invisibility, yes, but not both at the same time. Thus the Rogue, who *can* be simultaneously stealthy and climbing walls, isn’t put out of a job by the invisible flying Wizard any more.
Having said that, for some classes the concentration concept is overused and ends up making their spellcasting overly weak. A prime example is the paladin, who has very few non-concentration spells, at least at lower to mid-level. Spells like the level 1 smites really shouldn’t be concentration, as they are already not very powerful and concentration means they don’t work with more important spells like Bless or Compelled Duel. For the Warlock the fact that Hex is a concentration spell and the very staple of his existence, makes any other concentration spell nearly useless.
So, yes, concentration is a useful new concept. But I think it is currently applied to too many spells and could be better balanced.