Answer:
There are trivial similarities — both were tall.
Somewhat more significantly, they both had a reputation for honesty over and above other presidents. “Honest Abe,” and George “I cannot tell a lie” Washington after he (supposedly) chopped down a cherry tree.
But the most important similarity is this: Both of them were successful war leaders who had a strong instinct against war and in favor of peace. Evidence for this is:
Washington — Urged by Hamilton and Jefferson to support England and France, respectively, Washington forged a policy of neutrality and avoiding foreign wars. Presidents Adams and Jefferson eventually followed suit.
Lincoln — In his one term as U.S. Congressman, Lincoln voted against the Mexican American War, a war so popular that Lincoln failed to get re-elected because of this vote.
In this respect, Washington and Lincoln were not unique — they were joined in this quality by Franklin D. Roosevelt (and some might add Woodrow Wilson).
Ironically, these were among the most successful war leaders in history, but their biggest wish in each case was to have a country, or a world, without war.
One last thing: Washington and Lincoln both tried to be uniters, not dividers. George W Bush said it, but Washington and Lincoln meant it.