Each blackjack variation has its own rules, odds and strategies. Before playing it is recommended to take a look at the rules of the specific variation.
Dealer soft 17
There are two slightly different dealer strategies. In the "S17" game, dealer stands on all 17s. In the "H17" game, dealer hits on soft 17s; of course, he stands on hard 17s. (In either case, the dealer has no choice; he must or must not hit.) The H17 game is substantially less favorable to the player. Which game is customary depends on locality. Las Vegas Strip rules are about equally split.
Number of decks
All things being equal, fewer decks are more favorable for the player. (This is true for basic strategy players, even without card counting.) In fact, all things are not equal; multi-deck games almost always have otherwise better rules than single-deck games.
The player may "give up" and get back half his bet, before taking any other action. (In some places, this is posted, "surrender is available", while in other places, it is available, but it is not posted.) In some cases (15 vs. ten, 16 vs. 9, 16 vs. ace), this is slightly favorable. In one important case (16 vs. ten), it is definitely favorable.
Resplit to nn
(In other words, the option to split exists for a two-card hand from a split the same as the first two cards.) The simplest rule is "resplit infinite"; this means that the player may continue to split so long as he receives same-value cards and is willing to put up the additional bet. More typically, the rule is "resplit to four." These rules are practically identical, since even four hands is fairly unusual.
In general, after splitting aces, the player gets only one card even with the above rule. With this rule in effect, an exception is made: if the second card is an ace, the player can resplit. (This is a favorable rule.)
Double after split
In other words, the option exists to double for a two-card hand from a split the same as the first two cards. Generally, the player should play a hand after a split the same as the first two cards. However, this rule does slightly change which hands should be split in the first place.
Double on 10 or 11 only
Often called "Reno" rules. (Also seen is "double on 9, 10, or 11 only".) It is annoying to many players because doubling soft hands is considered part of the game.
Altered payout for natural
In some places, a natural pays 6:5 or even 1:1. This is the most unfavorable variation, increasing the house edge significantly more than any other player restriction.
Dealer wins ties
This is catastrophic to the player, though rarely used in standard Blackjack. It is sometimes seen in "blackjack-like" games.
Five card charlie
With this rule, the player always wins when five cards have been drawn without busting.