I recently re-watched The Avengers and I'm curious, at the end when Tony Stark takes the Nuke through the portal, he seems to hold onto it for a long time. Is there any reason he held on for so long? All I can think is it's to make sure it's accurate and hits it's target.
In contrast to some of the comments made above, I'd add this factor: Bewilderment.
The events of Iron Man 3 lead us to believe that Tony is having a midlife crisis, in part informed by the fact that he is the sole human to found himself piloting a warhead into the swelling ranks of an invading space-army, Dr Strangelove style.
Tony is haunted by the events, and wakes up in a cold sweat after dreaming of them, breathless. For a character with such natural bravado, the episode has clearly left some kind of mark on him, some psychological damage that manifests in anxiety attacks.
It's actually a very humanising moment for the franchise. He seems somewhat overwhelmed by the sheer scope of the Chitauri fleet, and in speaking to Pepper he directly articulates his overbearing feeling of helplessness in light of "what's out there".
As he passes through into Universe-parts-unknown, he does seem to hesitate longer than neccesary; and I'd argue that he is momentarily crippled with fear, and uncertainty. This is seemingly backed up by his eventual degradation into "A hot mess".
It also nicely sets up the concept of a wider, breathtakingly vast universe for Guardians of the Galaxy, which can't do much harm.