Skill composition is a funny thing. On the one hand, a team without the right composition of skills at the right level is going to cause problems. On the other hand, most leaders tend to hope their team can figure that out by themselves.
Some of this can be due to budget, some can be due to aggressive schedules that don't allow for training and skill development.
Either way, great teams have the right composition of skills at the right level, and you pay in quality and loss time without it.
Cross-Functionality
Aside from being a buzz-word, cross-functionality within a team makes a massive difference. Cross-functionality doesn't stop with technical functionality.
If your endeavor needs support from legal, InfoSec, design, etc then set up the team with folks who are also set up to help.
Dependencies kill productivity and creating dependencies in team composition is avoidable.
This is also highly related to How To Break Work Down