I'll say that it's no harder to prepare for than any other RPG.
The hardest thing for me to prepare is an instance of an in-depth mystery or plot. I can work one up, but with great difficulty, since I find it hard to strike a balance between an absurdly simple mystery, and one that is so scant in clues the players have no idea what's going on.
As for prepping game stats, which is the most common complain heard, I give advice exactly as I gave the last person to ask this question, about 7 or 8 months ago:
NPC's - the most important thing to remember about these NPC's is (1) distinguishing characteristics, (2) what they know or COULD know of use to the PC's, and (3) what are they GOOD at. Unless the players are being especially contrary, these guys and gals don't need base attack bonuses, every stat score worked out, and every skill point and feat allocated. I know their names, classes and levels, exceptional stats (above 13 only), and their characteristics. All this takes one row in a printed spreadsheet.
FOES - Almost the opposite. If the Players are in an adventure against a goblin tribe, let's say, or facing a natural beast like an owlbear - I need to know hit points, BAB, attacks, and damage. They don't need to know that goblin #12 secretly practices macrame when not near his peers. If a skill comes up, I assume that they will have ranks in that equal to their hit dice, with no exceptional bonus points. Frankly, the players will NEVER know nor care if a monster has one or two more points or less points in a given skill than they could have. Why? Because of all the unknowns. Said monster COULD have a slightly exceptional ability, or a skill focus - all these things are unknown until they become important.
I have NEVER found a player who was concerned if a monster or NPC had a +3 or a +2 to his jump skill. If the player is concerned about minutiae like this, then I.M.O. they are not enjoying the game enough to focus on what's happening, rather than what's NOT happening. It's not anyone's fault that they are not focusing - just that it's a sign that something needs to change.
BIG FOES - if you have a recurring villain, such as a noble or a wizard, or a necromancer with his own barony - these people need to be fully fleshed and statted out. In fact, I find the most fun in statting out these beings, because they are the closest thing I as a DM have to playing an PC in the game. I try to play them by the rules, I do not cheat on their stats, or in no other way give them undue knowledge or story immunity - But I play them dirty, I play them to the fullest, and I play to win. The PC's, when they win the day, feel that they have won a major victory, as opposed to simply following along in the story.
If the Players play their characters simply, and do not play to win, they they can LOSE. I do not take exception to this. They can always run in the face of losing, they can most always get resurrected at a level penalty (certain churches are always willing to resurrect adventurers, assuming they sign the standard indentured service contracts), and they could always roll up new characters at one level lower. The feel they get when they win against the foes is pretty qood, or so they tell me.
In summary, prep is what you make it. I do not see D&D, any version, as any harder or easier to prep than any other RPG. Anytime you are playing a consistent world with continuity, there is prep involved. And if continuity is not required (what I rather snarkily and unfairly call the "Voyager" style of campaign), then almost NO prep time is needed.