https://en.bitcoin.it/wiki/LitecoinRedundancy
Besides a faster first confirmation, Litecoin does not provide any other features over what Bitcoin provides. Because of this lack of innovation, some believe Litecoin is unlikely to match or surpass Bitcoin's value or user-base. It remains to be seen if the use of a different mining algorithm and faster block times add sufficient value for Litecoin's long-term survival.
Not Silver to Gold
Some argue that Litecoin cannot make sense as "silver to bitcoin's gold", because Bitcoin itself is both gold and silver: While in the long-run, the BTC unit may be too valuable for everyday trade ("gold"), there are other, much smaller units that can just as well serve the purpose of "silver" while being naturally/automatically "converted" to/from BTC.
Vulnerability to mining monopoly
Similarly to Bitcoin, Litecoin can be attacked by an entity that can match or exceed the hashrate of the network. Such a "51% attack" becomes more difficult to launch and maintain as the hash rate of the network grows. However, this argument posits that Litecoin is designed to be inefficient on all common computer components (both CPUs and GPUs) meaning that a malicious entity need only produce a small batch of specialized/custom hardware to overtake all the commodity mining systems combined.
Memory bandwidth refutation
Some attempt to refute this by arguing that scrypt is not designed to be inefficient, but is instead designed to be highly dependent on memory bandwidth. Since the high-speed cache RAM on modern processors already takes up most of the die space, no sizeable improvement could then be made by creating custom chips. If we accept this argument we then estimate the cost of attack utilizing GPUs that are avilable today.
To do so we start with an estimated cost of hardware at $400 per megahashes per second and the October 2013 Litecoin network hashrate of 30 gigahashes per second. The total amount of equipment necessary to match and takeover the Litecoin network via 51% attack is then an estimated $12M USD (or about 45,000 AMD HD 7950s).
Pump and Dump Scheme
According to some, one or more of the aforementioned reasons imply that Litecoin has no future potential, and therefore effectively functions as a "pump and dump" scheme, rewarding those who get in sooner at the expense of those who adopt it just before it finally fails (and are left with nothing).
Additionally, people often complain that the Litecoin community misrepresents it in other ways, such as portraying "faster block times" as if it makes transactions faster, and scrypt as if it is resistant to ASIC or FPGA hardware, in order to pretend Litecoin has value and inflate its value.
It's important to note, generally these critics do not think that Litecoin/Blockchain currencies are pump and dump schemes "per se"; but rather that the existing network effect of Bitcoin, combined with the lack of meaningful differentiation between Litecoin and Bitcoin and Litecoin's adoption of a "designed to fail" proof-of-work algorithm; that Litecoin is bound to fail in the end. Bitcoin does not suffer from these "flaws" and therefore does not fall under the "pump and dump" scheme, according to this argument.
