Calculate karma based on retweets
Here are some tips:


Use $.getJSON() to get JSON.

Also, name the first parameter json for clarity.

Note from documentation for jQuery.getJSON()


  JSONP
  
  If the URL includes the string "callback=?" (or similar, as defined by
  the server-side API), the request is treated as JSONP instead. See the
  discussion of the jsonp data type in $.ajax() for more details.


More information here

Old Code:

var url = 'https://api.twitter.com/1/statuses/show/' + id + '.json',
    karma; 
$.ajax({
    url: url,
    type: 'GET',
    dataType:'jsonp',
    crossDomain: true,
    success: function(data) {
        display_karma(data.retweet_count);
    },
    error: function() {alert('fail');}
}));


New Code:

var url = 'https://api.twitter.com/1/statuses/show/' + id + '.json?callback=?';
$.getJSON( url, function (json) {
    display_karma(json.retweet_count);
}).error(function () {
    alert('fail');
});

Eliminate commented code. Use a source control system like git, svn, mercurial to keep track of changes.

Here's a good start for it

Old Code:

function start_get_karma() {
    var id;

    $('.tweetContainer').each(function(index,el) {
        //id = $(el).data(el, tweet_id);
        id = '248988915661410304';
        get_retweet(id);
    });
}


New Code:

function start_get_karma() {
    var id = '248988915661410304';
    $('.tweetContainer').each(function (index, el) {
        get_retweet(id);
    });
}

Function calls are expensive, so use a basic loop instead of each() when appropriate.

Old Code:

$('.tweetContainer').each(function (index, el) {
    get_retweet(id);
});


New Code A:

for(var i = 0; len = $('.tweetContainer').length; i &lt; len; i++){
    get_retweet(id);
}


However, there's a problem. Making multiple calls to get_retweet() with the same static value doesn't make sense.  So just make one call.

New Code B:

if( $('.tweetContainer').length ){
    get_retweet(id);
}

Don't declare variables if they're only used once.

Old Code: 

function display_karma(retweets) {
    var id, karma,
        el =$('.tweetContainer');
        id = '248988915661410304';
    karma = calc_karma(retweets);
    el.find('.tcPoints').text(karma);
}


New Code:

function display_karma(retweets) {
    var id = '248988915661410304';
    $('.tweetContainer').find('.tcPoints').text(calc_karma(retweets));
}

Have variable names give a hint to the type.

retweets sounds like a function or array. Try naming it retweet_amount or something similar.

Final Code:

function calc_karma(tweets) {
    return (tweets * 10) + 10;
}
function display_karma(retweet_amount) {
    $('.tweetContainer').find('.tcPoints').text(calc_karma(retweet_amount));
}
function get_retweet(id) {
    var url = 'https://api.twitter.com/1/statuses/show/' + id + '.json?callback=?';
    $.getJSON( url, function (json) {
        display_karma(json.retweet_count);
    }).error(function () {
        alert('fail');
    });
}
function start_get_karma() {
    var id = '248988915661410304';
    if( $('.tweetContainer').length ){
        get_retweet(id);
    }
}