It was quite obvious that Alice didn't return because of Debra's role on That 70s show... and the show was on for quite a few years.
With Frank Jr coming back in S10, the specific plotline wasn't great, but the rest of the episode was brilliant. And I also echo that the reason we didn't see more of Frank (Phoebe's Dad) is because of his role in S5, it wasn't that great either and as someone already said, I think it would have been a waste of an episode to have him return again. I'm glad that he didn't actually turn up at Phoebe's wedding, all the other friends at that point were more like family than any of her actual family would ever be. I mean, seriously, Ursula could have at least made an effort? I know she didn't invite Phoebe to her and Eric's planned wedding, but I would have thought, Phoebe would have invited her. However, we don't know, she could have invited her, but she just didn't come.
With David, I guess Hank just didn't have a lot of time in that time, I know that during 2004, he has 4 films on the go (which is when the majority of S10 would have aired) and he was starring with Jennifer Aniston in Along Came Polly in that year. It's a shame, we didn't see enough scenes with Hank and Jen in Friends though.
Now as for the "laughter". I hear on many shows how they add laughter, but on the S10 DVD extras, I was watching the interviews with cast and crew, and in some cases, the opposite was done in Friends. Apparently when Janice entered Joey and Chandlers apartment in S5 as Ross' date, the real audience actually wooped and cheered for much, much longer than we see on the show, they had to cut down a lot of that.
Same with S6 first episode, I'm pretty sure I heard Jennifer Anniston improvised, "This is not a marriage, this is the worlds worst hangover", the line was supposed to be a simple unfunny, "Stop saying marriage". But the audience reacted well to what Jen said that they kept it in... even though they had to cut out a lot of the audiences reaction.
And again, I know I've said this before, but I love TOW The Truth About London and the ending scene that Jennifer was actually unaware about. Her freakout was actually real