Main reason I'm not a fan is twofold.
1. More matches between lower ranked teams. I don't think this can be disputed, given we will have a knockout round with 32 teams, so even in the knockouts we will have more Denmark or Switzerland vs Cameroon or Tunisia type matches.
2. Favouring negative tactics and general fairness. Seed 1 beats seed 2/3. Seed 2 draws with seed 3. Last match, both teams are happy to draw as seed 1 goes through as winners and seed 2/3 go through in second. It avours the team playing in the final match much more as they know what they need to do. That obviously happens to some extent now but not to the same extent imo.
Positive is more knockout matches, but I don't know if twice as many knockout games will be good when there are less exciting matches to get there. In all honesty I suspect they will try and bump it up to 64 at some point, doing the rugby world cup groups with 8 groups of 5, two qualify would at least have enough jeopardy to penalise defensive football. The Euros group stages are worse for allowing 3 teams of 4 to qualify from a group. 40-50% of teams qualifying from a group should avoid too many dead rubbers, but making draws less valuable.