Never seen a serious problem with understanding spellcasting, except forgetting concentration (but it doesn't come up that often at level 1)... I would
never tell a beginner "you shall not play a wizard until you've first learned how to play something else", it's horrible advice IMO and does a bad service to the game.
One thing I did encounter problems with, is managing the whole set of character abilities. Beginners
do often have a hard time keeping in mind everything that their PC can do. And there are 2 (related) reasons for that:
(a) 1st level characters already have a large amount of abilities
(b) character sheets are designed from the point of view of
building a PC instead of
playing a PC
I typically try to mitigate problem (a) by encouraging players to play human characters
or let them freely choose the race from a narrative point of view, but still use the human stats for all races (I also tried the middle ground, and kept 1 or 2 features per race instead of the full set).
With relation to problem (b) I designed my own character sheets (link below). They aren't perfect, and I still update them every now and then as I notice issues while playing. But at least they try to address a couple of typical problems with character sheets: having more stuff on them than the player really needs, being difficult to use proactively (i.e. as a source of ideas about what to do), helping you keep in mind reactive abilities. I really think that this is because they are designed more to help the player
fill the sheet at character creation than
use the sheet during the game; a typical character sheet flaw is to organize features by
source (e.g. class features vs racial features vs feats...) instead of organizing them by
usage (e.g. combat/exploration/rest/social/downtime).
http://www.enworld.org/forum/rpgdownloads.php?do=download&downloadid=1415
Edit: I forgot to mention another issue... beginners can really struggle with the general amount of rules, if the DM gives them the impression that they actually
need to know a lot of them. This is very commonly the result of the DM explaining way too many things before the game starts. Now, you might think you are helping the players by teaching them how to play the game, but the truth is that with beginners you might very well not helping them at all, but instead making them feel unsuited to play. Don't do it! Don't explain a rule until such rule is required to be used in the game. Minimize the time spend preparing the first game session and creating a character, and jump into the game as early as possible.