as you said, you can no longer contribute... not an issue since you can save elsewhere.
typically you cannot change anything inside the account. all the investments stay put like they were the day you left.
i guess i dont see any harm in leaving it there, but i think most money minded people would prefer to at least have some control over what they could invest in. rolling over to a traditional ira, you could like still invest in all the funds you currently do... but you would have the flexibility to change things around as your risk tolerance or the economy changes.
my mom is about to retire. when she does, we will be rolling over her 401k and a smallish pension plan out to a trad. ira.
if i leave my current company before i retire (i am 36, so this is probable) then i will almost certainly rollover into trad and roth ira's (i have trad and roth 401k accounts at work)