Seeing the poll choices it is strange to see Federer 09 with less votes than his other years. Yes Federer was clearly better and more dominant a player, regardless of competition, in 04-07 than 09. Yes he greatly benefitted from Nadal's problems, health and otherwise, in 2009. However all that said he still faced stronger overall competition than any other year he was on top, even shockingly without Nadal really as much a factor from RG onwards, and still ended with 2 slam finals and all 4 slam finals and clearly #1 by years end. Nadal was great until the Madrid semis, so about 5 months worth of top Nadal. Del Potro was at his all time peak this year. Since Del Potro didnt start playing great until Nadal stopped, we could mesh them together as 1 player of the year I guess. Djokovic was pretty good, better than 2010, and more consistent than 2007 or 2008 even if he had trouble rising for the biggest wins. Murray was really good this year. Roddick was resurgent and excellent this year, stronger overall probably than any other time in his career outside of 2nd half of 2003/2004. Soderling was a force now obviously. Davydenko was at his peak, a consistent contender, and ended the year with a huge win at the WTF. So he had Rafael Potro (again combining the January-May Nadal with June-November Del Potro), Murray, Djokovic, and a close to peak Roddick at the very top, and a strong second tier of challengers with Soderling, Davydenko, and others.
04 you had peak Roddick and peak Hewitt. Arguably peak Safin too. Decent depth. So pretty good competition.
05 Roddick and Hewitt both declined visibly in their levels, and Roddick still ending #3 playing worse standard of tennis than he did in 2009 ending #6 is telling. Safin was injured most of the year after Australia and never a contender again. Nadal was precocious and rose to a deserved #2 after dominating clay and doing well in smaller tournaments on all surfaces, but was not a slam contender outside clay yet.
07 had Djokovic and Nadal rising in their levels. Federer contemporaries were barely contenders anymore though, apart from Davydenko and maybe Roddick.