Negative Variations in Blackjack
Negative variations in blackjack are those rules changes which negatively affect the house edge, from the perspective of a player. If by adopting a blackjack rule in a particular game of cards hurts your chances of winning, then this produces a negative variation. Knowing the rules a gambler should avoid lets those gamblers pick the most advantageous game of cards in the casino. This skill is the equivalent of finding the best pay table for a particular version of video poker or, to put it more starkly, finding the poker table with the worst players.
Blackjack Pays 6-5
The negative variation to avoid above all others is the table with the sign which says "Blackjack pays 6-5". This one single rule increases the house edge by $1.39 for every $100 wagered. The traditional payout for a blackjack tends to be 3-to-2. In this game, every time you get lucky enough to hit a natural 21, you're paid 1 and 1/2 times you're wager. Using the 3:2 rule, if you bet $10, then you receive back your original wager, along with $15. If you were playing with the 6:5 rule, though, then if you bet $10 and got a natural 21, you would only receive $12 as a reward. Over the course of a long session of blackjack, the money you lose playing 6:5 blackjack becomes substantial. What makes this worse from your perspective is this is a total giveaway. Since the blackjack is the luck of the draw, you're simply agreeing to receive less money when you get lucky. Avoid the 6:5 table.
Blackjack Pays 1-1
I should mention that a table with a sign saying "Blackjack pays 1-1" should be avoided like it's the plague. This negatively affects you chances even worse. If you play with these rules, the house edge increase $2.27 for every $100 wagered--or $0.88 worse than the 6-5 table. This situation is less frequent (though more outrageous). Unless you're playing a game with specific reasons to accept this absurd payout (like double exposure blackjack), you should avoid these tables at all costs.
Blackjack Pays 7-5
When the blackjack pays 7:5 odds, this negatively affects the house edge 0.45%. For every additional $100 you wager, you'll lose an extra 45 cents. This may not sound like much, but since the "blackjack pays 7-5" is probably a short walk away from the "blackjack pays 3-2" table, you should keep walking the perimeter and see if you can find a 3-2 table of blackjack. This may not be possible in out-of-the-way casinos, such as a Native American casino in a state with few other gambling options. Just for the record, the order of preference should be 3:2, then 7:5, then 6:5, then 1:1 payouts for blackjack.
Dealer Hits on Soft 17
When the rules of the game allow the dealer to hit on a soft 17, this increased the house edge by 0.22%. The dealer hitting on a soft 17 is a common rule that can be hard to avoid in many casinos, even in the online blackjack industry. If you happen to see a game which advertises "dealer stands on all 17s", this is the preferable version of the game to play.
For those new to blackjack, a "soft 17" is the name for a hand which equals seventeen, but has an ace in it. Since the ace can be either a "1" or an "11", this is a soft total, because the dealer can't bust on it. In this case, a game where the dealer is allowed to (perhaps) improve their hand without any danger of busting is a bad proposition for you.
Multiple Deck Blackjack
Some list double deck, four deck, five deck, and six deck blackjack among the positive variations in twenty-one gambling. Since single-deck blackjack was the original form of the game and remains the most desirable form of blackjack for players, I view single-deck blackjack as the baseline game. Since increasing the decks always lowers your payback and increases the house edge, I have to view the increasing deck sizes as negative variations in blackjack. Among these, four different deck sizes are notable: two deck, four decks, five decks, and six decks.
Double deck blackjack increases the size of the deck from one 52-card deck to a deck of 104 cards. This game is played from the shoe, instead of dealt from the dealer's hand. The cards are also dealt face-up, instead of face-down, and the players cannot touch their cards. In the double deck blackjack, the house edge increases 0.19%. Most players still consider two-deck blackjack to be a pretty advantageous game. Card counters still get use out of a card count on a double-deck. This becomes harder as the size of the deck doubles again.
Playing with four decks increased the house edge by 0.06% and makes card counting a much more different proposition. Playing with five decks increases the house edge by another 0.03%, while playing with six decks increases the house edge by 0.02%. Some casinos even have 8-deck blackjack, but 6 decks is usually the point where the advantages gained from seeing cards go out of the deck no longer matter. New blackjack players should remember that every time the game deck increases by another deck of cards, this hurts their chances of winning that much more. Increasing the deck size in this way was instituted to thwart card counters, but this innovation has hurt the odds of the average blackjack players using basic strategy, too.
Player May Double on 10-11 Only
Some casinos restrict when a player can double on a hand, which is a devious way to shave a little off your expected return (increasing the house edge). If you don't double and keep a "10" or "11", 30% of the hands you build are going to receive a "10" and therefore be strong hands of 20s and 21s. Even if you receive an "8" or "9", the hand is still pretty strong. On the other hand, a split of a 10 or 11 is likely to produce to weaker hands, so doubling on these may not always be a great proposition. That's why this rule negatively affects the house edge by 0.18%.
Player May Double on 9-11 Only
A similar rule which lets you also double on a 9 has half the affect on the house edge, but still increases the house edge by 0.09%. Once again, such rules limit your strategic options, so they provide the casino an advantage over gamblers.
Players May Not Double after Splitting
When the casinos restrict your ability to double after splitting, this harms your chances of winning at blackjack. In some cases, splitting increases your chances of hitting two big hands instead of one. Imagine you're dealt two aces. You would want to split these aces, since you have a chance of hitting and receiving a 10, which would give you the coveted 21-hand. Since 30% of the cards in the deck have a value of 10, on 60% of the scenarios where you split aces, you're going to receive a 21 on one of those hands. That's a natural time to double your bet, to take advantage of a positive expectation scenario. Barring players from making this move hurts your game odds.
Split to Only Two Hands
Some casinos allow you to double after splitting, but restrict the number of times you can split. One common rule is to allow you only to split to 2 hands, but stop any further splitting after this. This rule increases the house edge by 0.14%. Another common splitting restriction is to allow a split only to three hands. In other words, you can split a hand twice, but no more. While this rule is the least damaging of all variants rules in blackjack, this still lowers your expectation by 0.01%.