Wedding wishes should be written either formally, casually, humorously or religiously, and should match the couple to whom they refer. For most couples, it is okay to write the term "congratulations" on the wishes, but a formal or traditional couple may be offended by that and would prefer "best wishes."
Formal wedding wishes concentrate on being both timeless and romantic. Examples include wishing someone a lifetime of love and happiness, wishing a couple happiness as they build a new life together, and expressing gratitude at being invited to this important and joyful day.
Casual wedding wishes concentrate on messages that are both short and sweet and suited to those uncomfortable with formalities. These include simple messages like "best wishes," "I am happy for you," and "hugs and kisses."
Funny wedding wishes should avoid sounding snarky, sarcastic or insulting. These should conform to the couple's sense of humor and might include thanking them for the free alcohol, quoting movies, and humorously reminding them of the minutiae of cohabitation such as the need to scrub the toilets.
Religious wedding wishes should conform to the religion of the couple and should not be more formal than the couple. These might include asking God to bless their marriage, quoting appropriate religious passages and mentioning that you will pray for them.