Wow ... well, I personally think the rules of chess are perfect for the game. There are plenty of other "chess-like" games with slightly different rules and pieces and if you're unhappy with how chess is played, look at one of those. But of the ones you've listed here (most of which are actually fairly major changes to the game in terms of strategy), I'd have to say I'd go with #14..
Hypothetical Rule Changes
Allow me to clarify- I like chess just the way it is, but IF you've ever felt like changing the rules just a little bit, for just a little while, perhaps to get you thinking outside of some box or rut you're stuck in... that's when you come to this topic.
I would choose number 19.
As someone who learned Xiangqi before chess, I know the sting of having stalemated someone, only to learn that it is a draw.
(In Xiangqi, stalemates aren't draws. Like in checkers, if you can't make a move, you lose.)

A rule whereby if my flag falls, I get another hour on the clock.
(It's already like this at the start of the game. If only it was like it later on!)
@jdthompson: On 6 you are correct and this makes essentially no difference - assuming both players see the game the same and use their passes at wise times. However with 5 there is an enormous difference, as you can pass repeatedly. This rule effectively eliminates zugzwang and turns things like K+R v K into draws by threefold repetition.

25. promote to the piece who's file is shared.... ie a8/R, b8/N, c8/B, d8/Q,
and you get another king for the e-pawn

WanderingWinder wrote:
@jdthompson: On 6 you are correct and this makes essentially no difference - assuming both players see the game the same and use their passes at wise times. However with 5 there is an enormous difference, as you can pass repeatedly. This rule effectively eliminates zugzwang and turns things like K+R v K into draws by threefold repetition.
But if white repeatedly passes, so can black. At that point it becomes a stalemate due to inactivity.

add the rule from shogi if you capture an oppoenet's piece you can use it but in your color, or have pieces like in shogi. Bughouse chess you can. it makes the game easier and hardier depending on the way you use it.

I'm surprised nobody has yet mentioned:
Pawns can move backwards
or
Pawns can capture straight ahead
or how about:
Prisoner exchange: "I'll give you a piece back if you give me back a piece of equal value"
And since we're talking about alternate chess rules, has anyone seen the thread on 3-way chess? Now that's some seriously modified rules.

WanderingWinder wrote:
5. You can pass your turn on any turn 6. You can pass your turn but only once per game
Extend the concept of passing your turn to going backwards (in time) i.e. undoing 1 or N moves. (This would be very easy to implement on computer-based chess). Could you suggest what the notation would be for each of your rule changes? For this one: 26. -25, -24.
En passant, only castling once and the initial double-move for pawns are clearly bodges anyway - which mar the pure simplicity of chess moves.
pvmike wrote:
I thought you could promote to a piece that hasn't been captured
You're right.
#2 is to remove that and limit promotions to captured pieces only.
jdthompson wrote:
WanderingWinder wrote:
@jdthompson: On 6 you are correct and this makes essentially no difference - assuming both players see the game the same and use their passes at wise times. However with 5 there is an enormous difference, as you can pass repeatedly. This rule effectively eliminates zugzwang and turns things like K+R v K into draws by threefold repetition. But if white repeatedly passes, so can black. At that point it becomes a stalemate due to inactivity.
Well, it's not a stalemate, but it is a draw. And that's the major point of the rule change.

The game Nomic (http://en.wikipedia.org/wiki/Nomic) consists almost entirely of rule changes. If, as an alternative to one of your moves in chess, you could change one of the rules, chess would become more of a memory challenge - trying to figure out which rules are in force for the current move.

paul211 wrote:
... Just to make chess a little more interesting as I love my knights firstly, clue for you, and bishop second, especially in the end game, I would allow knights to move like bishops and bishops to move like knights at any time. ...
Two things puzzle me:
- you claim to like knights more than bishops especially in the endgame BUT I bet your preference for knights over bishops depends upon how the pawns are positioned in the endgame.
- If you have 4 composite knight-bishops, although they can move in more directions, it reduces the number of different pieces by one, thus making the game simpler.

There's a site for this called http://www.chessvariants.com
The first one was "AAUUGHH! Chess", whereby there's a 1/18 chance of the rules switching to another in a list of variants.
Another I saw was where the King and horsies (time penalty lol) are swapped. There would be two Kings and only one Knight. The rules of the game is to mate the knight instead, a more elusive piece.
I got an idea for a game. Each chess player has their own chessboard, but can't see the other player's board. At the start each side gets to place all of their pieces anywhere they want. The rules are changed so that instead of moving the pieces, you drop bombs on square locations. Like G4. The other player will call out either a hit or miss.
The game ends when both players get bored of playing.
If you could make one minor change to the rules of chess, what would it be? Here are a list of possibilities (by all means this is not exhaustive, feel free to post your own) and then I will keep track of how many votes each suggestion gets. This is just for fun and discussion, so don't sweat it too much. Alright, here's the list
1. Get rid of the en passant rule
2. You can't promote to a piece that hasn't yet been captured
3. You can't promote to Queen
4. You can't promote to anything but queen
5. You can pass your turn on any turn
6. You can pass your turn but only once per game
7. Black gets to move twice on his first move
8. Castling queenside moves the king three spaces and the rook two
9. You can castle through check.
10. You can't castle
11. Instead of promoting, pawns just turn around (would need a way to mark this)
12. There is no check or checkmate. If the King is captured, then the game ends after the next turn. If in this turn the opposing king is captured, the game is drawn. Otherwise, the side which captured the opposing king first wins.
13. There is no check or checkmate. Play continues until one side is out of pieces.
14. Castling is legal if ever the king is on e1 or d1 with a rook on a1 or h1 and nothing in between or attacking any of the squares that the king would be in.
15. You can castle out of check.
16. Pawns can capture like they normally move (this is actually a huge change)
17. There is no check or checkmate. A side loses when they have lost both king and queen.
18. Knights can't jump (another major difference)
19. If you stalemate your opponent, you win instead of drawing
20. As your move on any move, you may place one of your opponent's pieces which you've captured onto any empty square from the second to seventh ranks. Note: this is like a 1-player bughouse and will require duplicate sets.
21. The starting position is shuffled, i.e. Chess960
22. Pawns cannot move two squares on their first move.
23. Time penalties are incurred for referring to any of the pieces incorrectly (i.e. Calling a knight a horse or a rook a castle)
24. Players can place any piece of theirs which has been captured on an empty square on their own second rank as their move on any move.
And my personal vote goes to 25. You may spend any of your moves to permanently remove one of your own (non-king) pieces from the board.