Why mobility is a problem

It is indeed fun to have mobility on a champion . It makes parts of the game more fluid and opens up possibilities for plays.

So, why is it a problem?

Well, with Riots new policy of skill shots (which is fine on some things, don't get me wrong) these mobile champs get to essentially simply sidestep another characters damage. This is especially true for many burst mages who used to counter mobility, but are now being more frequently reworked with less reliable stuns and skill shots. Mages also tend to have more utility and hence more ways to catch dashing opponents.

Case in point take rework veigar. If you take him into battle against an evenly skilled ahri or leblanc you are probably going to lose. Why? Well all they have to do is dash past your Q and then you literally must one shot them with your ultimate to survive. Not even going to mention W. Going to land a long delay skill on a rapidly moving target... unlikely.

I use this example because it highlights how few things counter mobility except to have more mobility. It becomes a slippery slope at that point because the two other counters beside raw mobility are giant burst (kill them before they can escape) or hard CC. No one wants a game full of the counters either, which devolves into either being instantly blown up or locked down and unable to fight back. Riot also has started giving infinite slows (hi kalista and new ashe) as a counter to all the mobility, but then it also makes any champion that actually has to chase a target progressively worse until you must be building righteous glory or have chilling smite (like trundle, skarner, and volibear with flat move speed increases and no dashes).

Another issue to touch on briefly is they are less fun to play against because they are overbearing and oppressive. If they get ahead at any time you will never escape them again almost ever . Even tower diving is easier because they can dash out. This was one of the reasons akali was so heavily nerfed last season.

The conclusion, mobility needs to be special and an exception rather than the rule. Right now we look at Kassadin and we don't think, "omg that is a teleporting champion!" Instead he gets piled in with a dozen others and is basically a weaker leblanc. Same thing with Rengars ultimate. Might have been special when he was released, now it is just another dash that even gives a warning sign taking the place of an ultimate.

Finally, mobility has to count towards a champions overall "power budget." It's ok if they can fly around the map, but they shouldn't be able to insta kill people as they do it ( no dashing drive by's if you know what I mean). Let kassadin be crazy mobile for example, but then just leave the ult damage low or pure utility. Xerath by contrast should be allowed higher damage and longer slows or stuns rather than trying to homogenize less mobile mages into skill shot reliant inconsistent cc oriented champs who use up all their power budget in their damage.

Because let's face it, lots of very mobile champions have huge damage and cc to boot.