The best answer comes Scripturally. Today, the painters of the Middle Ages have given us a picture of Jesus as a grand, handsome man. In fact, the Bible tells us the opposite - Jesus was very plain and normal looking. Let's look at some verses to support this. Emphasis added.
Isaiah 53:2 (ESV) tells us the Messiah wouldn't be "beautiful" on the outside:
For he grew up before him like a young plant,
and like a root out of dry ground;
he had no form or majesty that we should look at him,
and no beauty that we should desire him.
John 5:13 (ESV) tells us how easily Jesus slipped out of a crowd:
Now the man who had been healed did not know who it was, for Jesus had withdrawn, as there was a crowd in the place.
Now, taking these two verses in particular in to account - Jesus was a plain looking Jewish man (or perhaps, ugly, to the Jews of the time). That said, the Jews of the time were expecting a king who would conquer their adversaries (this is simply what I've heard, I do not know the Biblical basis for their unbelief except that He "didn't fulfill prophecy" and that He just "made them mad"), hardly the appearance one would expect for the Messiah. A plain man?
That said, it's not hard to believe Jesus looked in a way that would not easily distinguish Him from those who followed Him. We also know through Scripture they came for him at nighttime (John 13:27-30). The use of torches was commonplace at this time - hardly enough adequate light to see what you're looking for. Simply put, Judas identified Jesus with a kiss as it would be easier to identify Him.
Even then, Jesus had to identify Himself to the soldiers who came for Him.
Then Jesus, knowing all that would happen to him, came forward and said to them, “Whom do you seek?” They answered him, “Jesus of Nazareth.” Jesus said to them, “I am he.” Judas, who betrayed him, was standing with them.