Numerous factors make altcoins great. It matters what new technology they are bringing and what pain point are they solving. Case in example:- ethereum's smart contracts. So, in order for an altcoin to be valuable to the people buying it, it needs to have cutting-edge technology that somehow improves the way people transact. Also, the market's trust in an altcoin makes it valuable. More the trust, more the people invest, hence the altcoin's market share increases. And vice-versa.