I've played Diplomacy for a while now, getting a bit bored by the AI -- not that I'm all that great against stronger AIs, but some exploits (static defenses that the AI never goes around) get boring.
That's why people play online multiplayer PvP. Once you get into it, you'll probably never want to play against the AI again. Online multiplayer also offers you real team games, such as 4v4 and 5v5, where teamwork and coordination can sometimes come into play. Larger team games can also add new elements of strategy that you won't see in games against the AI. I suspect that once you get up to speed, you'll conclude that multiplayer PvP is more interesting, more suspenseful, more intense, and more satisfying.
Now I'm wondering how I would like PvP online. I just read this forum for a bit and I got two impressions, or questions: 1) are there enough players around?, and 2) will I find players on my (approximate) skill level? (just for reference, I can beat 3 or 4 Hard AIs in a FFA, but only by using diplomacy to keep some of them peaceful -- so that makes me an advanced noob, I guess?)
Even if you had been playing vs. AI since 2008, you'd still be a noob against people online, and that's OK. Everyone was new once. Do you understand all of the game's basic mechanics? Do you have a good knowledge of all the tech trees, the different races, their ships, and their strengths and weaknesses? As a new guy online, you're going to get thumped for your first 50-100 games, but if you hang in there and try to learn, you'll eventually get up to speed. Try to enjoy the process of becoming tougher and tougher and an increasingly big pain-in-the-ass. One thing you can do is watch game replays and study the pro players.
(1) The number of people online is low, but there are enough that we can have several 4v4 and 5v5 every day between, say, 3 pm - 11 pm U.S. Eastern time. You will find yourself having to wait for games to open up though. Normally when one game ends, they'll be enough people available to play the next game.
(2) There are still newish players around, but they get mixed in with the pros. So in a 5 on 5 we might have 3 or 4 newish players of varying degrees of noobishness. Unfortunately, you will just end up getting thumped for a while as you learn the game.
Alright, second part of this question: assuming I'm going to play PvP, what's the biggest changes in playing style I need to get used to?
You're going to discover that real human opponents don't do silly, suicidal things that the AI does such as suiciding on starbases for you or allowing you to easily kill capital ships. Experienced human players will be more difficult than Unfair level AI because they make much better spending decisions. The good players will expand quickly. Also, you may not be very far away from an opponent, which means that you could get rushed early on. So if you are near an enemy, you need to focus more on military and on making a big fleet very early on. You want to get a bunch of your long range frigates out ASAP--enough to kill an enemy Sova (that has come to 70% embargo your home) or a Vasari starbase as it builds. Getting rushed can be frustrating, but it's easier to deal with as you gain more experience.
The other big thing is that in team games your responsibilities and challenges may change depending on your starting position relative to your allies and enemies. You could start off safely wedged between two allies, in which case you are in an "eco" spot and your job is to build a rich economy and to then feed credits and resources to select allies. You could end up in a situation where you much attack or at least occupy an opponent who's next to you so that another ally of your doesn't get doubled. You could also end up wedged between 4 opponents with 2 on either side of you in a challenging situation called the "suicide" spot in which case you're just hoping to survive and be a pain-in-the-ass.
I play SP at normal speed, but I guess online it's played faster?
Yes. The basic settings are: Locked Teams, Pirates off, Normal fleet size (this can vary), Quick Start on, Faster for all speed settings.
We have to play at Faster speed to reduce the game times. Also, experienced players prefer that speed and the game will slow down later on anyway once large numbers of ships are in the game. In case you're wondering, one of those 4v4 or 5v5 games typically takes between 1 hour 15 minutes and 1 hour 45 minutes to play, with more competitive games lasting a little longer. (How is that? Unlike single player FFA, you don't have to kill every enemy yourself; you match up more 1-to-1. Also, when the losing players see the writing on the wall, they call it "good game" and quit, sparing the winning players the boring mop-up motions and allowing a new game to get started.)
Also, are static defenses ever build online, or ist worthless?
The most common items are repair bays, which are very useful, especially for TEC who can upgrade them. Phase Jump Inhibitors (PJI) are also common and useful and force you to really think about what you're doing before you jump into an enemy gravity well. People also build starbases, though Vasari starbases are most (often very) common followed by potentially explosive TEC starbases (er, bombs). Turrets are also useful but they have to be properly and intelligently placed. Later on in the game some Vasari players will also build hangers with phasic trap. TEC players are liable to build Ogrovs to kill enemy starbases and static defenses. Expect Vasari and Advent players to attack enemy starbases with bomber spam.
As a general rule, having a big and properly-constructed fleet is your best defense. A fleet can move, but static defenses cannot. However, static defenses don't consume any fleet supply and they don't raise your taxes. You'll want to use static defenses selectively at chokepoints and places where you think you might be attacked (the planet next to your enemy's planet).
Finally, I usually manage only one large fleet, but I guess if you defend mainly via mobile forces, you'll need more than 1 fleet, yes/no?
You'll probably want to keep everything in one large fleet. What's most common is to have a large fleet and then a small colonization fleet (a capital ship, maybe a colonizer frigate, and perhaps a couple light frigates or long range frigates to help clear out militia). Often you will see wandering, unaccompanied capital ships that players will use to attack undefended planets.
The online multiplayer PvP game totally rocks. I hope you enjoy it. Sins online multiplayer PvP is a mostly unknown and overlooked gem in the RTS pvp genre.