I think a shorter season is better. One of the problems with U.S. Series is that that they generally tend to go for 22-episode runs and it can result in things being unnecessarily stretched out.
Having said that, I don't think Agents of SHIELD has that problem. I just think the U.S. should follow my country's (UK) more successful format of between six and thirteen episodes per series/season. It allows for a tighter story arc and reduces the danger of stagnation.
If not this format, then they should do more U.S. dramas like Season 4's three-serial structure (Ghost Rider/LMD/Framework). In the UK we're only just about to enter the Framework storyline but so far these mini arcs have added a breath of fresh air to the show.