TOR also has benefited incredibly from the server mergers and larger-pop servers. It is a game that really needs that mass of people on the same server. There are lots of small group quests, and being able to quickly get a group in local chat makes a huge difference. As well, being able to fill out your group with companions makes grouping, especially for the [HEROIC 2+] quests, a lot easier.
Making the LFG server-only was an interesting move. I'm not totally sure that it's a big difference maker, but even after just running LFG for a week as DPS, I'm beginning to see familiar faces. Groups are mostly good and nice, which matches my experience with WoW as well.
I even ran an instance today, Lost Island, which came out in 1.2, with a group of people who had never run it before. We wiped on all the bosses, but generally figured out everything and eventually completed the instance successfully. It was a great experience, and one that I haven't had in a great while.
Funniest moment in that run: we're fighting a boss with a knockback that drops us into the lava. There's a force field bridge that goes to the boss's platform. The healer theorizes that maybe we're supposed to get knocked up the bridge, so he stands on it. Fight starts, bridge disappears, and the healer falls into the lava.
There have been a couple bad groups. Last time I discussed the Group Finder in WoW, I facetiously remarked that maybe it was the healers who caused problems in groups, and since I always healed, I never saw the drama queens and always had good groups.
After running instances as DPS, I'm inclined to think that maybe there's a grain of truth in that. I had one group where the healer got upset at our strategy, and ran away from a boss fight as it was started, causing a wipe. The healer then left the group. After waiting a few minutes in LFG, we pulled out a companion healer and beat the fight.
(You can't do that often, this time it was because both DPS could CC the two main adds, greatly reducing the damage flying around.)
That's really the only negative experience I've had with the Group Finder in TOR. For the most part, the Hard Mode flashpoints are fairly easy, comparable to Wrath or post-nerf Cataclysm heroics.
That's not to say that the Group Finder has made everything perfect though. There's still a fairly long wait for DPS, on the order of 20 to 30 minutes. As well, the Group Finder dumps out outside the instance entrance when you're finished, usually leaving you a fair distance away from where you were before starting. Travel in TOR is a bit of pain, with multiple load screens, so this can get annoying.
The other unfortunate interaction is between TOR's daily group quests and the Group Finder. Each area with dailies also has one or more repeatable daily group quests. It's usually fairly easy to get a group for these quests. But the problem is that you have to leave the Group Finder queue in order to group up for the dailies, starting the queue all over again when you finish the group daily.
The reason behind this behaviour is fairly straightforward. You don't want people leaving a group in the middle of something because they got an instance group.
What would be nice is if the Group Finder "paused" if you joined a group, and then re-inserted you at the same position when you left that group. Like if you start at position 300, eventually get to position 100 after 20 minutes, then join a group for a daily. Group Finder removes you from the queue. After finishing the daily and leaving the group, Group Finder re-inserts you at position 100, and the queue continues, rather than having to start over at position 300.
But still, these problems are relatively minor. The Group Finder works, and has made TOR a much better game.