Litecoin is extremely vulnerable to a 51% attack due to the vulnerability of its pools to being DDoS'd, and due to there being significantly less mining capacity needed to reach 51%.
Legit question: How is this different from Bitcoin in its youth?
I'm going to do a little calculation for my own sake... and probably to my own LTC-holding peril!
The current network hashrate is 5.6 GH/s. Litecoin Scrypt is said to be (and anecdotally has been found to be) 1000 times more difficult to hash than SHA256 (serialization, memory hardness). So to compare to Bitcoin, that is the equivalent processing power/equipment to 5.6 TH.
50% would be 2.8 GH/TH worth of equipment, all done without ASICs. The most reasonable way to accrue this kind of power is through renting a botnet. Let's assume the average person's computer (as part of a botnet) could do 100 kH/s (most would be CPU, some would be gaming rigs). To get 2.8 GH/s, that is a 28,000 slave botnet. From various articles online I have seen that such a botnet might cost $200 x 28(thousand) = $5600/hr (for a "world mix").
That is not an unreasonable amount to a nefarious character who has a high profile target in mind. But botnets are fickle things. A world mix would see lots of downtime at the bot-per-bot level, such that you may get 80% efficiency. Also, I may have been overly generous with the 100kH/s average estimate.
A lot of people know why and it seems quite a lot know why it wasn't a good idea too.
95% of this thread is people talking about confirmation times. It is my view that the security issues are founded but may be somewhat overblown, but I don't know much about the mechanism and logistics on carrying out timing attacks on the network.
Disclaimer: my crypto holdings are about 30% LTC, 70% BTC.