Are there any _severe_ drawbacks of bitcoin justifying all the alternatives? There exist quite some alternatives to bitcoin that are partially based on it, and I'm sure their creators all are convinced they improve upon something. But are there any serious drawbacks in bitcoin that can or will probably never be fixed?
I don't think there are severe drawbacks of Bitcoin. Certainly none that other coins are solving.

However, Bitcoin has certain properties that could be replaced with some other. Most altcoins to just that. Popular properties that altcoins change are:


replace PoW with something else


Some altcoins do not use a PoW mechanism or use a hybrid mechanism. PeerCoin (originally named PPCoin) was the first altcoin to use the Proof-of-Stake mechanism, which basically requires miners to hold a certain amount of currency in order to mine new. PeerCoin uses a hybrid of PoS and PoW. Nxt is an altcoin that uses pure PoS.


the PoW algorithm


Bitcoin uses SHA-256, which is a hash algorithms that almost entirely requires CPU cycles to calculate. This means that it is easy to create custom hardware to perform this calculations. There are many altcoins that use Scrypt as the PoW algorithm. Scrypt is more memory-intensive and therefore harder to implement in custom hardware. The first altcoin to implement Scrypt is Litecoin. Primecoin requires miners to calculate large prime numbers as a PoW mechanism.


the total amount of coins that can be mined


Bitcoin has a 21 million coin limit. Some altcoins raise this limit, such as Dogecoin, others lower it, such as 42.


the rate at which new blocks are created


Bitcoin blocks are mined at an average rate of one block every 10 minutes. Many merchants require that a transaction has been confirmed by several blocks before they accept it. With a confirmation time of 10 minutes (in fact, the recommended amount of confirmations to wait for is 6) can be quite cumbersome in certain use cases. Litecoin has a confirmation time of 2.5 minutes.


the time it takes before the maximum amount of coins is created


Block rewards usually have a halving time. Every halving time the block reward halves until it is too low to represent using the maximum precision and it becomes zero. With Bitcoin it will take until 2140 until the block reward becomes zero. Some coins shorten this period. Sometimes they shorten it so much that the creator (who mines first, of course) is accused of pre-mining because he can mine very fast and easy in the beginning of the existence of the coin.