Blu-ray's main drawback is cost.
Prices for players are still generally over $200 and movies cost about $25. While the one-time cost of a player isn't that bad, the cost of building up a new Blu-ray library really adds up. At least it's possible to pick and choose which movies you "buy Blu," since every Blu-ray player can also play standard DVDs.
Another downside is that the number of titles on Blu-ray is still much smaller than DVD.
There are currently about 970 Blu-ray titles available, compared with more than 90,000 on DVD. Depending on your taste in movies, you may only find a few movies you actually like available on Blu-ray.
Load times are longer then DVD's.
When Blu-ray first came out, load times were unbearable; it could take more than 3 minutes to load a movie. Since then, players have gotten much faster, but they still don't compare with the speed of loading a DVD. While simple Blu-ray movies can load in about 20 seconds on a good Blu-ray player, movies with complex menus still take close to a minute and a half to get to the actual movie, regardless of the player.
Lastly, if you start buying Blu-ray movies, you may get frustrated that your new movies won't work in places where you only have a standard DVD player.
For example, if your bedroom only has a DVD player, you won't be able to watch the second half of your new Blu-ray Disc from the comfort of your bed. Or if you have a car with a built-in DVD player, your new Blu-ray Discs won't work there, either.