Yep, it really comes down to a good single player experience. Does it tell a coherent story and actually have a reason for all the weapons in the game or is it just a convienient pile of crap tacked on after a multiplayer slugfest was created. It seems that many games do have a good single player portion, but that portion is really short. Usually these games have four difficulty settings: retard, normal person, fps veteran and uber ninja able to dodge the 100 percent accuracy headshots coming in at you from every direction.
But what if a game is 100 percent multiplayer? Then it's a MMORPGER! Seriously though, looking at WoW, you can actually play it as a single player game all the way up through end game content. You don't need to make any groups or run any dungeons. Any elites you can just ignore. PvP is entirely voluntary on all the decent servers. If you look at other MMORPGERS like FFXI, you have pretty much a 110 percent multiplayer game. You are stuck in a party for every leveling excursion. Any new area you are going into will be with five of your pals. Everyone needs to be in their top end gear and spells for every encounter, buffed up with the right food and a balanced party. While you might be able to solo areas quite a bit lower level than you and farm items for gold by yourself, most everything you do will be in a group. Is the grind FUN? It's a pretty fun game with a very different pacing than WoW. I enjoy it, and come back to it from time to time. But I still like that human interaction is entirely optional for most of WoW. And I LOVE that you can turn off channels of chat (I'm looking at you barrens chat!).
You totally got it wrong when you said buying a game with multiplayer and not playing the multiplayer was like buying the orange box and not playing portal. More like not playing team fortress 2. Portal is the short (but fun) single player distraction in the orange box. Personally, I never bothered playing TF2, because I don't really give a damn about the multiplayer. I bought the orange box for HL2 and it's episodes, and portal was a bonus. And I love each of those games. It really does make the orange box a good value. I'm sure some people bought the orange box for TF2 and ignored the rest of the awesome games in it.