IMO when it comes to shounen shows, you kind of have to think early on "am I really going to want to watch this for [possibly] a decade?"
Taking characters and style into account is very important. A show that consistently degrades in quality, or a show where you couldn't give two shits about what the cast does on their off days (filler) is not a something that you should watch.
Also, having a favourite character to invest in is always a good thing, because it just makes you anticipate the moments when they get to kick ass. I guess one thing to take into account, is to figure out if it's a show that overvalues its protagonist and gives little screentime to the "sidekicks" or if they simply stand on the sidelines and praise the protagonist. Shows where each character really contributes to the main genre, action, comedy, whatever, tend to shine.
Obviously you need to check out a show first, so a full 12 episodes is nice. Personally I probably won't follow Fairy Tail, but I was a big fan of Groove Adventure Rave so I wanted to give it a shot.
For reference, I'm knee-deep in One Piece right now. I'm following the dubbed Collection sets and I'm on episode 145 and can't get enough. One Piece is known for pretty consistent quality (there were some small issues in early episodes) and the cast is pretty damn diverse. Each character has a ton of personality, even side characters (the dub is also easily one of FUNimation's best, ever, ranking up alongside FMA and Baccano!) I definitely dig the format of venturing to new lands every so often, the islands have as much personality as the characters and it keeps filler-ish stuff from just taking place in generic anywhere-lands, and the "off days" I mentioned are pretty fun because of each characters' quirks. While Luffy definitely tends to take on the high priority targets, each character gets a large amount of screentime and episodes that give lots of attention to a specific character are pretty common. PS: Robin is the best.