I admit I haven't done too much digging yet, but if you read this, a US military assessment of him on Wikileaks, it does mention the contacts with al-Qaeda and other terrorist groups, but it doesn't mention any involvement whatsoever in violent actions. Is it enough to be jailed under normal circumstances - yes, of course, it's undeniably aiding and abetting terrorists, though I think it's a stretch to say it makes him personally a terrorist any more than, say, an Irish-American funding the IRA during the Troubles is a terrorist. Would it get him more than the twelve years in jail that he's already served - that, I'm not so sure of.
What's sure is that many people with much worse records have been released in the past as conflicts came to a resolution - and that people with much worse records have even been welcomed and fĂȘted by the US or other Western governments, I doubt I need to provide examples. So yes, I can grant that Obama negotiated badly, but not that this guy was so horrible that he should never have been released, as a lot of people seem to be saying.
Afghanistan is negotiating with the Taliban. Pakistan is negotiating with the Taliban or related groups. We really didn't need this deal to tell us that the war's goal of destroying the influence of the Taliban had failed.
And do I think they're going to sit quietly in Qatar - not quietly exactly, no, but I do think Qatar doesn't want its reputation blemished by letting them slip away. Of course this is a morale-booster for the Taliban, which is why it should have been done in exchange for more serious concessions.