Actually, he didn’t. Angel wanted revenge on Sahjahn, so he cast a major spell to make him tangible (previously, he had been completely and permanently insubstantial-couldn’t touch anything even if he wanted to) so that Angel could kick his ass for what he did to Connor. Sahjahn, as it turned out, was a lot tougher than Angel, but at the last moment, Justine (a freelance vampire hunter who had been tied in with the kidnapping of Connor in the first place) cast a spell on him that imprisoned him in a big vase.
Justine later framed Angel for the murder of Holtz, Connor’s surrogate father, which led to Angel being the victim of some illegal dumping off the L.A. coast.
Also, Charles Gunn joined the cast at the end of the first season. Previously, he had been the head of a vampire fighting street gang, before teaming up with Angel etc. to fight the big league evils.
Lastly, Cordelia is now psychic and partly non-human. Well, actually she may be 100% non-human, depending on how next week’s episode turns out. Briefly: halfway through season one, they offed Doyle, who was a regular cast member and a psychic half-demon. Somehow, before he died, he passed his psychic powers onto Cordy. However, being human, Cordy’s visions were gradually killing her. She was given the opportunity to lose them, but decided that she would rather keep them and help Angel for as long as possible. In return, The Powers That Be made her part-demon, so the visions wouldn’t blow off the back of her skull. Later, she was promoted to Divine Being, but apparently got demoted for meddling in the mortal world by helping Angel out of a tight spot. There followed some amnesia, Oedipal conflicts, and now we’ve got Big Bad Cordy and her Doomsday progeny.
Lastly, Wesley used to be Faith’s watcher before they canned his ass for letting his Slayer turn evil. He used to be comic relief, but later turned into a genuine badass after taking over Angel investigations while Angel was on a Brooding Sabbatical. Then he turned into a slightly evil badass after he kidnapped Connor and got shunned by the rest of the gang. However, he was the guy who dredged Angel back up from the sea floor, and now everything seems to have been forgiven.
Confused? Wait 'til someone tries to explain how Angel had a kid in the first place.