My daughter has a wish list but it is on the side of our fridge at home. When she was a 1, 2 & 3 I did send a little note about her clothes size (shirt & pants are different sizes) BUT that was only certian family members. I knew those people were going to ask anyway so I was just one step ahead of them, it was a little specail note paper I put in the invite not on the card itself.
At age 4 we put 'no gifts' but everyone brought items anyway, most small like a bell for a bike, puzzle, most knew she was big into tinker bell and yes those same family members that asked about clothes asked again.
Just last week was my daughter's 5th birthday, sent out invites with the basic info on it (what, date, time, place, RSVP) nothing else. Most gifts were low key like an outfit, coloring book, hello kitty stuff, tangeled, dress up clothes.
As someone else mentioned that putting up a wish list is micromanaging your guests gifts... I totally can see that I did that the 1st & 2nd year for both birthday & christmas. The past 3 years it has been awesome to see what people think to bring, we only invite family and my daughter gets to pick out 3 close families/friends to come.
If you do not know what to get them most web sites let you narrow down by age, gender and price range... it is so helpful, inculde a gift receipt and enjoy yourself. Also a gift card is just fine, it lets the kid explore that store or save up for something they really want.
At christmas time we do a name exchange between us siblings & their others so in this case we do make up a wish list of items between $0 and $40. One of my brothers always puts "surprise me" while most of us has a few things down, including gift cards.