I dont really see a need for this personally as the current lfm system is great for pugging. However, on the assumption that its going to happen anyway:
I'd like a preview screen of the group I'm about to be teamed into detailing what the quest is, what difficulty its on, and the current confirmed party makeup in the same overview style as you'd get from hovering your mouse over the lfm listing. A 30s to 60s accept or decline button would probably be in order to speed up the process when there are people on autojoin that fall asleep at the keyboard.
Speaking of afking. One of the biggest issues I have with automated grouping in MMOs is the inevitable abuse of the system by people who have no intention of contributing to the quest/raid/battleground/whatever. They use the anonimity of the autogrouping system to let them get groups they would otherwise not be able to get into. It is a very unpleasant experience to be in a group with said afkers, and quickly makes the game "unfun". Good examples of this were WoW's pvp queuing system around season 4-6 (can't really speak for the current system as i've not played in years). In a 40 man alterac valley team, you could easily see 10+ people sitting in the starting area doing nothing for the whole match, and the names would start to become familiar over time. If you left the match you'd get a leaver penalty (and someone else would be auto-teamed to fill your place) so you couldn't stop them from getting the completion. Our current LFM system has some protection against this, through the power the group leader has to simply decline the person applying, and to kick someone after they've joined if they were hiding behind anon. /squelch automates the decline process.
A more current example that I've experienced is StarTrek Online's fleet action system for STF missions. Players that use the autoqueue system are thrown together with no regard for "class" or ship type, and teleported into the mission. People who want to afk can do so with no penalty, beyond the detrimental effect on the fleet's ability to pew pew at stuff. Several of the missions have no ability to "fail" and be forced out of the mission, so you can be stuck in a mission where you dont have the firepower to succeed, but you cant fail and be put out of your misery. If you leave, you are flagged as a leaver and cant join other STF missions (even private ones that you personally form) for an hour. The icing on the cake is that rare loot from random kills is forced into a need or greed system on said autogroups. So said piker is merrily rolling need on everything of value too.
That ended up being quite ranty. tldr: I've seen autogrouping done very badly, and turned playing game, the object of which is to entertain, into a depressing, unfun experience that has left me angry at the product. Please be very careful how you code this feature, if at all.