Chekhov's Gunman: Before going to his first gathering, Graystripe hopes that Clawface and Stonefur will attend. Both later become major players. Also, Barley who was once a member of BloodClan, and the small apprentice Lionheart talks to at Fireheart's first gathering, who later becomes ShadowClan's medicine cat.
Kits call their parents by their names rather than by Mother or Father
Apprentices must travel to the Moonstone before becoming a warrior. This is mentioned several times in the series (though Fireheart and Graystripe are the only ones that actually do as far as the reader knows) and forgotten later on. After fans pointed this out to the authors, it gets Lampshaded in the later part of the second series by Leafpool, who comments that they seem to have left that tradition behind when they came to the lake.
Even she-cats not expecting kits are referred to as queens in the first book.
Earlier books have slightly different spellings than later books - Highstones is HighStones in the first book, "crowfood" is two words or at least has a hyphen, "Clanmate" is two words.
At Gatherings, cats of all ages tend to mix: examples include senior warrior Lionheart talking with some apprentices, and Fireheart sitting with a group of elders and medicine cats. In later books, the characters tend to stick with the cats their own age.
Mates aren't (for the most part) treated as major relationships in the first series, and it is mentioned that warrior fathers don't stay close to their kits (Crookedstar being a rare exception.) In the third series, Spiderleg is critisized because he doesn't want to play with his kits, and in the third and fourth series especially, everyone gossips about young couples and young she-cats talk about who they like and things like that.
Evil Is Hammy: Brokenstar until he is stripped of his leadship of ShadowClan. Later on Tigerclaw takes a level in hamminess after becoming ShadowClan's leader.
Genki Girl: Cinderpaw, until she gets hit by a car.
He Knows Too Much: Tigerclaw's primary reason for trying to kill Ravenpaw and Fireheart.
Heroic BSOD: Bluestar suffers a particularly nasty one after Tigerclaw's betrayal. It takes her two entire books to get over it completely... just in time for a Heroic Sacrifice.
He tells Bluestar to meet him by the Thunderpath, setting his scent markers too close to the edge. He hopes Bluestar will run right onto the road, not realizing how close she is, and be hit by a car. Cinderpaw ends up caught in it instead.
He tries to drown Fireheart, as mentioned below under The Uriah Gambit. Even Fireheart wasn't sure whether it was an accident or not until he noticed the way Tigerclaw was looking at him later.
Darkstripe gave Sorrelkit deathberries to eat; if Graystripe hadn't seen what happened, every cat would have just assumed she found the berries and didn't know what they were.
When an apprentice becomes a warrior, their name's "-paw" suffix is replaced with something more appropriate.
Brightpaw gets special mention here. When she is wounded and nearly dies, Bluestar gives her the name Lostface as part of her Rage Against the Heavens. Later, when Fireheart becomes leader, he renames her Brightheart.
Mentor Occupational Hazard: Bluestar, Lionheart and Yellowfang, mentors to Fireheart, Graystripe and Cinderpelt respectively, and all mentors to Fireheart in some way, all die.
Tigerclaw, suspecting that Ravenpaw knows he killed Redtail, sets him some dangerous hunting tasks, including at Snakerocks (named for the deadly adders that bask there in the summer... Ravenpaw actually manages to catch one!) and in ShadowClan territory.
During battle, Fireheart's pinned down and fighting for his life. He calls to Tigerclaw for help, but Tigerclaw just watches him; it's to his advantage if Fireheart dies since he believes Ravenpaw told Fireheart what he knows.
At one point, he orders Fireheart to cross a flooded stream, using only a spindly branch caught in the water. When Fireheart's halfway across and Longtail isn't watching, Tigerclaw tries to Make It Look Like an Accident by knocking the branch loose of the rock it's caught against.
Wham Line: And with a cold shiver of dread, Fireheart realized that the new leader of ShadowClan was Tigerclaw.