Nothing. I switched from Virgin to T-Mobile when it looked like I may be traveling internationally so I wanted a phone that would work in Europe. My coverage is a little better with T-Mobile and the ability to roam to other carriers in the event I am out of T-Mobile's coverage areas is nice. They had a better choice of phones too.
The only bad thing I would say with Virgin Mobile is about the roaming, but if you are sure you say in Sprints coverage area (in the US) then you probably won't have a problem. Other than that, I'll still recommend people look at Virgin when they are looking for new plans.
I have Virgin and agree with the points here. Basically roaming on the $35/mo plan is non-existent but I have never had an issue with my phone not working in the areas I live and visit (unless there was a tower down). Sprint's 4G coverage is very poor so if you expect that then good luck but my phone is not so I don't mind yet and can easily switch to another carrier like T-Mobile when I do.
The only bad thing I would say with Virgin Mobile is about the roaming, but if you are sure you say in Sprints coverage area (in the US) then you probably won't have a problem. Other than that, I'll still recommend people look at Virgin when they are looking for new plans.