I think the most important feature of any altcoin is its actual use in the real world. If this feature loses hold in the altcoin, the altcoin will eventually die out. Before it finally dies, it might simply transform into a pump and dump coin that has no real value in real life. Everything depends on the relevant real-world issue that the project, along with the coin, wishes to solve or provide an innovative and easy alternative to.

The popularity can be a big factor to that but also the price of bitcoin or the trend of its value can make a huge difference to the trend of the value of your altcoin, like ethereum for example, which has the similar trend with bitcoin's value.

Technically speaking, an altcoin has value for the same reason everything else has value - people are willing to assign that much value to the object.
When I am looking into investing in an altcoin, I personally look into the devs, the community and the technology in that order. Novel tech is always good but it doesn't mean anything if there are sketchy devs or no community to pack it.

The more altcoins are accepted as currency by mainstream vendors (ie. how Bitcoin is accepted at some restaurants, websites such as Overstock, etc) can add value. Also, features and mentions in crypto and mainstream media can aid its perceived legitimacy (even though press coverage alone doesn't necessarily mean it is actually legit/safe/valuable)

There are many factors, especially the price of Bitcoin, the de factor reserve currency of the Bitcoin world, price of the U.S Dollar, Innovation, energy put into securing the blockchain of a coin, and reputation issues such as public perception, scams, and media response. Dilution of the market means many alternative cryptocurrencies will find it hard to gain a foothold. But many have a cult following so have some value. Legal issues can vary the price also as discussed above.

In my opinion, some field make altcoin valuable such as:
+ Anonymity
+ Micropayment friendly ( i like ETH because of this )
+ Friendly mining
+ No premine
+ Have development team
+ Huge community ( important)

People in this community gives value to a token or coins. If you want your altcoin to be successful and have value, you will need to advertise it well and make sure that your campaign has what it takes for people to also believe that your altcoin will really be profitable. In this forum, many of the people are really willing to invest in alt's but in order for them to invest in your coin, the coin must be well presented, unlike the shitty coins.

I personally think that are different aspects that makes a coin valuable, simple put, it has to have a utility and must have users, so it has to build a community and I see some good projects fail to become successful because they look to fail in spreading the news about them in the right way and thru the right channels. They have/are a lot of specialists in their team but they poorly advertise or don´t feed information thru the pre and sale period, I even tweeted about.

You can either make it as a currency for an app or other functions. The price and popularity of coins highly depends on how you make use of it.

If you are seeing the fact that most of the altcoin was generated from the ICO, remember the ico has a fixed price to the token. And the price of the altcoin detemined by divide the total amount raised with the total supply of the token. The community give the more value to the coin. It's pegged with the total raised amount. The airdrop token based on the interest by the investors.

Main thing for me that I look for is a good community behind a coin that is important as well as the dev team being active and providing updates. Nothing worse than investing in a coin and having zero updates even if the updates are minimal they should update at least. Even if updates are bad those should be posted as well, the only time devs don't really post is when they are leaving and scamming you and regardless of whether you are doing that at the moment people will begin to think that you are scamming them. This is human nature those two traits are what I look for most and I know it doesn't have anything to do with tech but that is what I look for in a coin beyond specs. Anybody can have good specs the others are harder to come by and I appreciate them when I find them in a coin.