I think sharingan is limited to copying ninjutsus' that require seals, seeing through genjutsu and copying the form of taijutsu.

Ninjutsus' that require seald can definitly be copied where other ninjutsus that either don't require seals or are influenced by a bloodline cannot be copied. The rasengan doesn't require seals and is basically all about chakra manipulation and therefore can't be copied. Summoning does require a seal but it also requires a blood oath between the summoner and the summonee therefore summoning can't be copied.

Genjutsus are illusionary techniques that the sharingan can see through. Now the level of the genjutsu may have a bearing on the effectiveness of the sharingan or the level of the sharingan user may be a factor. As Itachi sais against Kurenai 'that level of genjutsu has no effect on me', this could mean that a more elaborate genjutsu could effect him or conversly that genjutsu would work against a less experienced sharingan user.

The movements of taijutsu can be copied but not the skill required to perform them. As we saw in the fight between Lee and Sasgay, even though Sasgay could read Lee's moves he didn't have the skill to either perform them or resist against them.

Or course these are just my opinions. Feel free to question my sanity.