Hit or Stand on Soft 17
This is probably the most common rule difference in blackjack games. A soft 17 is one that includes an Ace valued at 11. The hand can thus be valued at 17 or 7. 17 is the standard value that a dealer would normally be forced to stand on. Soft 17 can be a special situation, however, depending on the casino.
The house edge is higher when the dealer has to hit on soft 17 than it is when the dealer stands on that value. The difference to the house edge is around a 0.2% increase when the dealer hits soft 17. Many casinos use the hit soft 17 rule, and those that don't often make up for it with other rules, so be sure to look over the entire rule set. Never just use a single rule difference as the yardstick for an entire blackjack variant.
Rules for splitting doubles can be complex, but the more liberal they are, the better it is for the player. Often, split hands that aren't Aces can only be hit, and not doubled down on. If the player can double down on split hands, then the house edge is decreased slightly.
Being able to split Aces is especially important for the player, as some blackjack variants count Ace and 10-point card hands after a split as naturals, paying them at the more favorable odds for a blackjack. The standard rule is that split Aces receive 1 more card and then stand, but if the player can hit hands of split Aces, this reduces the house edge by 0.13%. If Aces can be resplit, then the house edge is lowered by 0.03%.
Essentially, the more restrictions a game places on your ability to split, the higher the house edge is likely to be. If you take a rules base of split up to 4 hands, no doubling on split hands, and Aces can be split but not hit or resplit, then you should be able to roughly work out how the house advantage has been affected by the rule differences. That standard base has an optimal house edge of about 0.79% when dealer hits on soft 17 and 8 decks are used, so most blackjack variants should fall below that. Those that are more restrictive with splitting are likely to have quite a high house edge for a blackjack variant.
Natural Blackjack Payouts
The standard payout for winning with a blackjack is to pay 3 to 2, which may not seem that much better than the even money payout on normal wins, but you'll be cheating yourself if you believe that. Some blackjack tables pay 6:5 on winning naturals, and sometimes just even money. This is the most important of the common rules variants to pay attention to, as the 6:5 payout increases the house advantage by 1.4% and the even money payout increases it by an astounding 2.3%. It's a subtle rules difference to the unwary player, so be on the lookout for lower blackjack payouts.
The Number of Decks
The standard number of decks most online casinos use these days is 8. The less decks used, the more advantageous it is for the player. As has been mentioned beware using a single rule as a yardstick for whether to play a game or not. Normally, when fewer decks than 8 are used, the casino will make up for it with other less favorable rules. Single deck games in particular attract disadvantageous rules like honey attracts flies.
If a casino offers blackjack with fewer decks and very few disadvantageous rules changes, then the house edge should be significantly lower than at other casinos. Get in on a game like that while it lasts, as casinos are known to occasionally release such games for a limited time, and then retract them once they have attracted new players to the casino.
No Hole Card
While the US tends to play blackjack with one dealer card face up, many blackjack variants in the rest of the world have no dealer hole card. This changes basic strategy, as the player has no information on the likelihood of an outright dealer win, which could put double down or split hand bets at greater risk. The effect is a 0.11% increase in the house edge if all bets are lost to a dealer blackjack, and no increase in house edge if only the original bet is lost to a dealer blackjack. Oftentimes, more complex rules are given for multiple losses to a dealer blackjack, and these all have their own effect on the house advantage. Games like Australian Pontoon are an example, but this is a fairly complex variant with many rule differences to standard blackjack.
Which Card Charlie?
Many blackjack variant allow players to win automatically if they draw a certain number of cards without busting. This will greatly change the basic strategy, as decisions on hitting a particular value will depend on the number of cards drawn to that hand. The addition of this rule is always advantageous to the player, but it is often accompanied by less favorable rules. Some variants will also payout 5-card Charlies at the higher natural blackjack odds of 3 to 2.
Other Common Rule Changes
There really is no limit to the rule changes in blackjack, but the previously mentioned ones are the most commonly encountered. Blackjack-like games and more unusual blackjack games like Blackjack Switch add in all sorts of unusual twists that can greatly change the nature of the game. For instance, in Blackjack Switch, players are dealt two cards and may switch the second cards dealt to each hand. Obviously, this rule is advantageous to the player, but it is countered by bizarre rules such as dealer push on 22.
This guide should help you to decide which standard games offer the best expected value over time when comparing common differences in rules sets. Perfect strategy goes a long way to helping cut down the house edge, though. If you are going to play a specific blackjack variant for any extended length of time, you should find its perfect strategy and memorize it or print out a strategy card for it. Playing in an online casino makes it much easier to refer to a strategy card, though, so consider joining one of the establishments featured on our online casinos reviews pages.