My solution was to play something as utterly different from CoH as I could possibly get. The answer, for me, turned out to be Dungeon Fighter Online, at least for the time being.
DFO got my attention by appealing to my love of retro games; in this case, old-school side-scrolling beat-em-ups like Final Fight, Streets of Rage, and Golden Axe. I often describe DFO to friends as the illegitimate love-child between Diablo 2 and Golden Axe. It's a fantasy universe, and character customization is kinda nonexistent unless you either shell out real money for Avatar items via the cash shop, or pay exorbitant amounts of in-game currency to players via the auction house to purchase avatar items. The community (I use the term loosely) rather sucks, full of e-peen-waving ten year olds, interspersed with frequent RMT-spam. I recommend that if you play it, play it with friends you know. Don't try to game with pubbies, most of them suck and just want to leech off you.
Still, the game's rather fun to play, on its own merits. The basic gameplay is very simple - you have four directional keys and four basic buttons (attack, jump, special, and special 2). All of your skills can either be activated by a direct hotkey (like hitting F1-F6) or by inputting a command sequence similar to Street Fighter - using the command sequences often gives you a small discount to mana cost and a small boost to recharge speed. The game consists of moving from room to room, killing all the enemies in the room, and then moving to the next until you reach the boss of the dungeon.
Where the game's challenge lies is in Combo Rate and boss fights. The game rewards you for killing several enemies at once, or killing enemies as part of a multi-hitting combo. The larger the percentage of the dungeon's enemies that you kill, and the more you kill as part of a combo, and the larger the combos you achieve, the better your rewards at the end of the dungeon. Boss fights typically present a unique challenge - either the boss will be a normal enemy of the dungeon upgraded with unique patterns and attacks, or a special, unique enemy with his own attack pattern. There's also quests, which basically consist of tasks to perform while in a dungeon - such as clearing it on a specific difficulty, or killing a certain amount of enemies of a given type, or collecting a certain amount of quest item drops from a specific enemy. You get quests from NPCs in town between dungeons, much like Diablo
Now, I will say that the game is much faster-paced than CoH, and frequently requires some twitch gaming skills, especially when strining together combos or fighting either other players or APCs (computer-controlled enemies that mimic a player character). Some classes require more or less of this than others (I highly recommend you avoid Fighters, Gunners, and Female Mages when first starting out) but just about all characters will require you to learn how to string your attacks together against a wide variety of enemy types in order to combo. If this is a problem, DFO may not be right for you.
DFO does run very well on low-end systems. My computer's a six-plus-year-old dinosaur, and it runs DFO quite nicely even on maximum settings. This isn't surprising, since it's almost purely sprite-based game that only uses 3D geometry for certain skill effects. Still, there's pretty much no modern computer that will have problems running it.
If you give it a shot, let me know - I'll be happy to help you through your early dungeons, maybe even set up a Mentorship with you so that you can earn a bit more XP from your dungeon runs.