Tina Fay is funny on Weekend Update, but I don't think her writing for the sketches is very good.
Jimmy Fallon has potential but a) They seem to have turned the whole show over to him, and b) He thinks he is so funny he constantly laughs at his own jokes. This is amusing occasionally, but not in EVERY STUPID sketch.
The rest of the cast is poor at best. Horatio Sanz is terrible, and the others are totally forgettable.
Another problem is that they repeat BAD sketch ideas, such as the stoner roomates and the idiot Bostonians.
The 2 biggest problems with the show, however, are:
1) They let sketches go on too long. Often they take one mildly funny idea, then go and base a 6 minute sketch on it!
2) They kiss up to anyone and everyone who hosts. They allow people to host that they SHOULD be poking fun at for pete's sakes! I bet you that if he agreed, they'd have Michael Jackson host and kiss his ass too.