interestingly, 'accidental magic' is actually somewhat more impressive than what just-got-their-wands first years can do.
Harry (in cannon) performs accidental magic during times of high emotion and when he's not able to cast spells (ie, before he has his wand or summer vacations). This is also true with Ariana Dumbledore, Snape, and Neville in each other occurrence of accidental magic in the series. In HP:MoR, wizards use the mana/muscle system for magic. It seems likely that magic can 'build up' if unused and release in an explosive (sometimes literally) fashion in the form of accidental magic. Furthermore, since nobody seems to intentionally use it, it must have a drawback; extreme draining of magic disproportionate to the effects seems like a logical effect.
Interestingly, both Tom Riddle and Lily Evans were able to consciously control 'accidental' magic as children. They both ended up the greatest spellcasters of their year. This would further reinforce the hypothesis that it extremely drains your 'magic' muscle, given that working out one's magic has been established to make one a stronger spellcaster.
Furthermore, since nobody seems to intentionally use it, it must have a drawback; extreme draining of magic disproportionate to the effects seems like a logical effect.
Lack of control seems like the biggest issue. 'Oops, I nearly killed someone' (Ariana), 'Oops, I didn't mean to make that glass disappear' (Harry), etc.
(The HPMOR discussion thread after this one is here.)
The previous thread is over the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it. The latest chapter as of 09/09/2011 is Ch. 77.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: