As you say, what could be difficult about a mate-in-2? Or, how can make mates in 2 difficult? The answer is probably one of the most common problem devices, the Grimshaw, named after 19th century composer, Walter Grimshaw.
The basic idea is that two opposition pieces are each stopping a mate so you make a move which has to be met by one or other of the pieces moving to a square where it blocks the other piece. These subtle first moves can be very difficult to find, although the level of difficulty is in the eye of the beholder. Three times world puzzle solving champion, GM John Nunn, almost certainly doesn't find the same problems difficult that I do.
Here is a simple "Grimshaw" mate-in-2:
[Title "White to play and mate in 2"]
[fen "8/B2K3Q/5p2/3k4/2p2P2/p6p/r7/b7 w - - 0 1"]
Knowing it's a "Grimshaw" and seeing as how black only has 2 pieces that can interfere with each other makes it much easier, but without that knowledge it would be frustrating tough.
Of course, top problem solvers know all about Grimshaw which makes 2 movers easy for them.