The Blues' problems this season have been caused by a combination of several different things:
1) Key players not having played during the lockout (Halak (1 game only), Elliott, Backes, Oshie, McDonald, Perron, Pietrangelo, Shattenkirk, Jackman)
2) Extended injuries to key players (McDonald, Steen, Tarasenko and Halak), -(3 of their Top 6 forwards, at the same time).
3) Langenbrunner and Arnott becoming too old, Langenbrunner hurt, the too old veterans replaced by rookies (Schwartz & Tarasenko) -leaving team with a lack of enough veteran calmness (especially with McDonald being out for a long time, and Nichol's game fading, similarly to Langenbrunner's.
4) Key rookies couldn't get integrated into the team's system, and lines didn't get enough prep time due to no pre-season, and the loss of the first 34 season games. Teams with set lines from last season have an extreme advantage over teams that are adding 2-3 Top 9 forwards.
5) Tarasenko's arrival helped spur a "slaughter" victory in the first game. He continued his good scoring in the first several games. The Blues started believing that he is a "budding superstar", and would help make their formerly ordinary "teamwork offence" a potent one. They felt that with their smothering defence/puck possession game of last season, plus their new, "potent" offence 9proven in the first few games, they'd be almost unbeatable. So, they felt (subconsciously) that all they had to do was step out onto the ice, and they'd win almost every game. They "forgot" to play Hitchcock's 2-way, defensibly responsible game. The fact that they were seemingly able to score very easily, led to another problem -leading the players (especially those playing for new contracts-but really including everyone) to TRY to add as many points as possible to their scoring totals). This led to much risky pinching, getting caught up ice and hanging back in the offensive zone, causing many more quality scoring opportunities for the opponents.
6) The results of #4 put added pressure on Halak and Elliott (in addition to their having had no pre-season and needing to use the first 2 months to "train"), causing them to lose confidence and not get in a "groove". So, Elliott has basically collapsed, and Halak has been very inconsistent.
7) The forwards' not getting back on defence, defencemen's ill-advised pinching and staying too long deep in the offensive zone (overall lack of team defence) and deterioration of the puck possession game (due to unsettled lines-addition of new players and lack of pre-season) has led to more possession by opponents in The Blues' defensive zone, and more goals against.
8) Arbitrariness of penalty calling by the referees hurts the formerly physical Blues more than the "finesse-style" teams. That has hurt The Blues' team defence and forecheck (especially the games of Backes, Oshie, and Polak, and has even hurt Pietrangelo's effectiveness).
9) ALL OF THE ABOVE- combines to lead to less confidence for the goalies, and overall team confidence. They suffer mental letdowns. Hockey is a TEAM sport. Mental attitude is VERY important. The Blues have a VERY FRAGILE mental condition related to confidence, which translates into lack of necessary confidence for sustained high-level play and sustained winning.
10) The Blues could still benefit a lot from having at least one, if not 2 difference-making scorers, who can manufacture offence on their own. Tarasenko may become one. But, he's not there yet, and his getting injured just sets that process back. They could use a top-scoring, super-all-around centre, as well(could be the difference-making sniper as well-but better if separate player, and good setup man).
11) The Blues need more size, weight and strength on the defensive line, especially a #1 shift crease-clearer (who can also bring the puck up ice and skate it out of trouble and make breakout passes). With the new "criteria" for calling interference penalties, their lightweight group of defencemen is more vulnerable than it was last season.
12) The Blues "sneaked up" on many teams last season (especially feeding on Eastern Conference teams-to pad their great won/lost ratio). The Kings "showed" the other teams that pressuring The Blues' weakest defencemen deep in their own zone would lead to many turnovers, and reverse The Blues' puck possession dominance, to lead to many scoring chances.
Summation: ALL of these factors have combined to keep The Blues from playing their successful puck-possession game, and therefoe, from getting the results they got last regular season.