It was the most popular year yet with over 210,000 people completing at least one puzzle. My coworkers were impressed with the conciseness and readability of WL several times (they were using Python and Rust). I completed all of the puzzles, but I used help on days 23 and 24. I made the leaderboard (rank 57) for part 1 of day 13.
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  • Day 9 - Use of MorphologicalComponents
    • ◼
  • Day 13 - Use of Graphics
    • ◼
  • Day 15 - Use of GraphDistance
    • ◼
  • Day 19 - Use of FiniteGroupData
    • ◼
  • Day 22 - Use of RegionIntersection
    • ◼
  • Day 24 - Use of Solve
    • This chart estimates the difficulty of each day’s puzzle based on the time it took the first person to solve it.
    Out[]=

    Day 1

    In[]:=
    Import["input.txt","List"]//Differences//Count[_?Positive]
    Out[]=
    1711
    In[]:=
    Import["input.txt","List"]//BlockMap[Total,#,3,1]&//Differences//Count[_?Positive]
    Out[]=
    1743

    Day 2

    In[]:=
    Times@@Fold[#+Switch[#2[[1]],​​"forward",{#2[[2]],0},​​"down",{0,#2[[2]]},​​"up",{0,-#2[[2]]}]&,​​{0,0},Import["input.txt","Table"]]
    In[]:=
    Times@@Fold[#+Switch[#2[[1]],​​"forward",{#2[[2]],#2[[2]]*#[[3]],0},​​"down",{0,0,#2[[2]]},​​"up",{0,0,-#2[[2]]}]&,​​{0,0,0},Import["input.txt","Table"]][[;;2]]

    Day 3

    In[]:=
    inp=ToExpression@Characters@Import["input.txt","Lines"];
    In[]:=
    gamDigits=inp//Transpose//Map@Commonest//Flatten;​​epsDigits=1-gamDigits;​​FromDigits[gamDigits,2]*FromDigits[epsDigits,2]
    Out[]=
    3148794
    In[]:=
    filter[list_,i_,isOxy_]:=​​With[{common=Last@Sort@Commonest@list[[All,i]]},​​Select[list,#[[i]]==If[isOxy,common,1-common]&]​​]
    In[]:=
    calc[l_,i_,isOxy_]:=​​If[Length@l>1,​​calc[filter[l,i,isOxy],i+1,isOxy],​​FromDigits[First@l,2]]
    In[]:=
    calc[inp,1,True]*calc[inp,1,False]
    Out[]=
    2795310

    Day 4

    In[]:=
    inp=StringSplit[Import["input.txt"],"\n\n"];​​numbers=First@inp//ImportString[#,"CSV"][[1]]&;​​boards=Rest@inp//Map[ImportString[#,"Table"]&];
    In[]:=
    timeToWin[board_]:=​​NestWhile[#+1&,1,n|->​​NoneTrue[Join[board,Transpose@board],rowOrCol|->​​ContainsAll[numbers[[;;n]],rowOrCol]]]
    In[]:=
    score[board_]:=numbers[[timeToWin@board]]*​​Total@Complement[Flatten@board,numbers[[;;timeToWin@board]]]
    In[]:=
    score@MinimalBy[boards,timeToWin][[1]]
    Out[]=
    14093
    In[]:=
    score@MaximalBy[boards,timeToWin][[1]]
    Out[]=
    17388

    Day 5

    In[]:=
    inp=StringSplit[Import["input.txt","Lines"]," -> "]//Map[ToExpression@StringSplit[#,","]&];
    In[]:=
    linePoints[p1_,p2_]:=NestWhileList[#+Sign[p2-p1]&,p1,#!=p2&]
    In[]:=
    countOverlaps[inp_]:=linePoints@@@inp//Catenate//Counts//Select[#>1&]//Length
    In[]:=
    countOverlaps@Select[inp,#[[1,1]]==#[[2,1]]||#[[1,2]]==#[[2,2]]&]
    Out[]=
    6397
    In[]:=
    countOverlaps@inp
    Out[]=
    22335

    Day 6

    In[]:=
    inp=Import["input.txt","CSV"][[1]]//Table[Count[#,i],{i,0,8}]&;
    In[]:=
    f[n_]:=(​​cur=inp;​​Do[cur=RotateLeft@cur;cur[[7]]+=cur[[9]],n];​​Total@cur​​)
    In[]:=
    f@80
    Out[]=
    386640
    In[]:=
    f@256
    Out[]=
    1733403626279

    Day 7

    In[]:=
    inp=Import["input.txt","CSV"][[1]];
    In[]:=
    calcAt[f_,p_]:=Total@f@Abs[Round@p@inp-inp]
    In[]:=
    calcAt[#&,Median]
    Out[]=
    355150
    In[]:=
    calcAt
    #+
    2
    #
    2
    &,Mean
    Out[]=
    98368520

    Day 8

    In[]:=
    inp=LetterNumber@StringExtract[Import["input.txt"],"\n"->All," | "->All," "->All];
    In[]:=
    inp[[All,2]]//Catenate//Map@Length//Count[2|4|3|7]
    Out[]=
    239
    In[]:=
    digits={{1,2,3,5,6,7},{3,6},{1,3,4,5,7},{1,3,4,6,7},{2,3,4,6},{1,2,4,6,7},{1,2,4,5,6,7},{1,3,6},{1,2,3,4,5,6,7},{1,2,3,4,6,7}};
    In[]:=
    solveRow[in_,out_]:=(​​correctOrder=SelectFirst[Permutations@Range@7,​​order|->Sort@digits==Sort@Map[inDigit|->Sort@order[[inDigit]],in]];​​FromDigits[out//Map[FirstPosition[digits,Sort@correctOrder[[#]]][[1]]-1&]]​​)
    In[]:=
    Total[solveRow@@@inp]
    Out[]=
    946346

    Day 9

    In[]:=
    inp=ToExpression@Characters@Import["input.txt","Lines"]//ArrayPad[#,1,9]&;
    In[]:=
    inp//BlockMap[If[​​AllTrue[{#[[1,2]],#[[2,1]],#[[2,3]],#[[3,2]]},p|->p>#[[2,2]]],​​1+#[[2,2]],​​0]&,​​#,{3,3},{1,1}]&//Flatten//Total
    Out[]=
    554
    MorphologicalComponents[9-inp,CornerNeighborsFalse]//Flatten//​​DeleteCases@0//Counts//TakeLargest@3//Apply@Times
    Out[]=
    1017792

    Day 10

    In[]:=
    inp=Characters@Import["input.txt","Lines"];
    In[]:=
    pairs=<|"("->")","["->"]","{"->"}","<"->">"|>;

    Day 11

    Day 12

    Day 13

    Day 14

    Day 15

    Day 16

    Day 17

    Day 18

    Day 19

    CITE THIS NOTEBOOK

    Advent of Code 2021 solutions in Wolfram language​
    by Michael Hale​
    Wolfram Community, STAFF PICKS, December 27, 2021
    ​https://community.wolfram.com/groups/-/m/t/2432017