Buying one just for the sign-up bonus
A large welcome offer looks attractive, but if you would not otherwise keep the card, the bonus is a one-time event. The card still needs to earn its keep every year after that. If the ongoing rewards and perks do not fit your spending, the sign-up bonus does not change the math.
Cards with complex redemption rules
Some travel cards require you to transfer points to airline or hotel partners to get full value — a process that involves understanding transfer ratios, award charts, and availability windows. If you are not going to manage a points strategy, the stated "value" of those points may never be what you actually get. Stick to cards that let you redeem simply: as statement credits or through a straightforward travel portal.
High annual fee cards you will not fully use
A card charging $500 or $700 a year is built for travelers who use airport lounges frequently, stay in hotels multiple times a year, and collect monthly credits. For a traveler taking one or two trips a year, most of those credits go unused and the fee is dead weight. Start with a $0 or $95 card instead.