I'll suggest Armageddon. It's free, no pay at all. Because of the roleplay and the nature of the code, the grind is only grindy if you want it to be grindy. It doesn't have to be grindy at all and in fact you can play without having ever bumped up a single coded skill at all. The coded skills exist more to support the roleplay, rather than the roleplay being based around the coded skills. However - if your character were to desire work as a crafter, it'd be in his best interest to actually -be- a crafter since the people who hire him would want to see some actual results at some point
That's just an example.
Combat can be enjoyable. It depends on what you consider enjoyable combat. I assumed I'd never play a combat-oriented character when I first started playing Arm, but eventually I did, and it turns out I rather enjoy it. But most of my characters don't spend all that much of their time using their combat skills. Again - it's optional, and depends on the character you create and its background and desires and goals and where he fits in the world.
Serious - definitely serious. It's pretty hard-core roleplay. If a new player shows up thinking he'll play Gandalf the Grey and demand respect and make it really REALLY obvious that he's only there to grief - then he's be treated as griefers by players and - if it doesn't stop - by staff. On the other hand, if a new player shows interest in the genre and theme of the game (a hybrid similarity between the Dark Sun D&D campaign and a desert world similar to Herbert's Dune series), shows an attempt to learn the syntax, and is willing to accept help when it looks like it's needed, he'll be welcomed and encouraged by pretty much everyone who plays.
Player base size: Definitely not massive, but not exactly sparse either. I play sometimes off-peak (eastern daylight, middle of the day or early morning sometimes), and usually find at least 20 people logged in when I type "who." During peak time (around 9-10 eastern time), there's upward to around 70, 80 people, and sometimes even over 100 if "something really awesome" is going on (otherwise known as an RPT). You might not necessarily run into any of those people - it depends on where your character is. If he's in a spot where people often congregate, then he'll eventually run into people. If he's in a clan that's currently populated by a lot of PCs, then he'll have plenty of interaction. If he's in an isolated spot though, he might not see a single other PC all day long. Even if there's 100 people logged in. The nice thing about Arm, is that if there are 100 people logged in, most of them are *not* idle or afk. Most of them are really there, watching their screen, and actively interacting with the game world.
Game client stuff: You can use their fairly primitive browser-based cleint, or you can use any telnet client and customize it however you wish. There are no graphics though - a couple of -very- primitive maps of the cities in-game (ascii maps). If you want fancy colors you'd have to configure them client-side.
No ducks or penguins. You can find their main website here: