I consider a game alt-friendly if it has a relatively short time between when I start a character and when I can meaningfully play with my friends. These games support the creation of a stable of characters, a toolbox. Games with slow levels or long grinds encourage having a main character, and alts definitely have a subordinate status. You may play your low-level alts with your friends, but that is clearly a separate activity where you are all messing around on alts. You would not bring one of those alts on a serious guild activity without taking it through the long slog to the level cap.
The three dominant factors in MMO alt-friendliness seem to be up/down-leveling, time to the level cap, the distance between effectiveness/nominal cap and the absolute maximum.
City of Heroes is probably the alt-friendliest game out there. Aside from task force restrictions, you can group any characters together, and you will all play the mission as the same effective level. City of Heroes pioneered sidekicking and went through several variations before settling on “heck with it, everyone can group with everyone.”
City of Heroes was more alt-friendly before it added alternative advancement. For most of the game’s lifespan, there was not much difference between a 1000-hour character and a newly minted level 50. There was some room for growth, and it was noticeable when someone had farmed all the way to full best-in-slot, but you rarely felt hampered by having half a team of alts. The effectiveness divide was level 22, the point at which you had most of your powers and could get single-origin enhancements. (With level 50 help, you can skip to level 22 in perhaps an hour.)
Contrast this with World of Warcraft, where you can keep grinding raid tiers and one level-capped character might have twice the damage of another. Asking for a higher gearscore is effectively asking for a higher level character, just the game tracks that advancement differently. And, of course, you cannot bring a level 20 for level 60 content at all. (Differences possible since I played.)
Guild Wars 2 adopts the alt-friendliness model in one direction with auto-downleveling. I still cannot bring my level 12 character to a level 75-80 zone (sanely), but we can gather levels 35, 45, 65, 80, and 80 and play in a level 35 or lower area. Guild Wars 2 also has a relatively quick time-to-cap, so I expect experienced players to have a stable of level 80 characters. Even the alternate advancement of gear accumulation has a relatively low cap, although there is a noticeable difference between green and orange gear.
EVE Online’s offline training is a different form of alt-friendliness. You can play one character while “leveling” another, but you cannot gain skill points on two characters at the same time (unless you buy a second account and gain skill points on both, which is great economics for your game if you can get players to do it). On one hand, this is encourages alts because you can keep advancing on your main while playing an alt. On the other hand, you have a clear division between any alts and your main, and any skill points you funnel into alts have a permanent opportunity cost to your main because there is no way to “make up” those points.
EVE is also interesting for its time-to-cap. It has absolutely no cap for overall skill points, only a theoretical soft cap that will never be reached because new skills appear faster than you can cap everything. Starting today, you will never catch up to a veteran. But you can cap individual skills, time-to-effectiveness is far shorter than time-to-cap, and only so many skills apply to a particular endeavor. I may never catch up to you, but I can be 90% as effective as you (at some things) in the time it takes you to earn your next rank 5 skill. At some things. It will still take me a year or three of training to fly a capital ship without just becoming a funny killmail, but you can contribute in 0.0 in your first month.
One thing EVE offers that you will have trouble finding anywhere outside Istaria or superheroes is that you do not need alts to switch roles. You switch combat roles by switching or re-fitting ships, but you can stay on the same character. You do not need an industrialist and a pilot and a gunner and a miner and a politician and a researcher. If you can have several characters under one account, why not just have several roles under one character?
(Digression: some games link mail and chat across characters so that you can at least find your friend on his alt. If I switch characters while we are talking, your reply will be auto-forwarded to the new character. Guild Wars 2 gives you one mailbox for your account, and all your characters are in the guild if one is invited. This is an alt-friendly feature. Of course, this makes it almost impossible to hide on an alt, but this post is long enough without that question.)
I have been calling this alt-friendliness, but in a broader view, this is easy grouping. Alt-friendliness means not needing an alt to play with your friend’s alt. If one member of our group wants to switch classes (or half, or all but one), that does not mean that either everyone switches or we are down a member for months. Letting you play with your friends is kind of the whole point of gaming.
A few times in The Lord of the Rings Online, I tried to start over with something other than the most popular class in the game. I still had my main for guild activities, but it was a months-long solo side project outside of scheduled events. I never got one of those characters to the level cap before I got sick of it and quit the game for at least six months. One of those was even an organized project, the Casualties of War kinship with a dedicated weekly group, which highlighted alt-unfriendliness: we needed everyone to play together or else levels spread and/or we were not on the same quest chains and/or steps. If everyone starts over but you end up spread across 8 levels a few months later, all of you are now alone. We gradually lost members and ended up in a small group limbo; even if we had had skirmishes to scale by level, they do not scale to all group sizes.
This raises another form of unfriendliness: quest steps. Nine-step quest chains are horrible for groups unless everyone intentionally runs them together. Otherwise, you are on step two, I am on five, and Bob is waiting for us to catch up for the final step. If I want to help your alt catch up to mine, I must repeat content for little to no reward, which is one way of bringing our levels closer together. Compare to City of Heroes where everyone gets a “mission complete” bonus for every mission, no matter who owns it or which story arc.
This is striking me more so outside MMOs. I have Torchlight 2 and Borderlands 2. You do not bring a level 40 to a level 10 game or vice versa. There may be some acceptable level gaps, but level ranges mean “trivial,” “suicidal,” or “let’s all start new characters again.” And it gets worse with more than two because good luck getting four characters’ levels to stay synched without making them “we only play these when we’re together” characters. And hope none of you ever want to switch characters.
Contrast with Orcs Must Die! 2, which has no character levels and goes straight to alternate advancement with a low cap. Unlocking and upgrading everything gives you more options, but you are 90% of the way to the effectiveness cap if you upgrade one each of floor, ceiling, wall, guardian, weapon, trinket. If your friend just got the game, you can play coop without worrying about levels or upgrades.
Alt-friendliness is also new player friendliness. If your game discourages your players from inviting their friends to play with them, your company deserves to fail.
This has been PvE focused. PvP games with level differences are often problematic and another topic altogether.