If I could add to lerincho and ntomlin:
1) Turbine has departed from PnP 3.5 rules in a number of places to implement the spirit of D&D in real-time combat (for example, using a mana pool instead of number of spells per day); there have been lively forum discussions about those design choices and how appropriate they are. So, there are many players with long histories of PnP, and many with none. [I grouped in a PUG once with a guy who had played with Gary Gygax ... off of photocopied rules (i.e. pre-box set)!]
2) you can have up to ten (10) characters to start with (per server!), so there is _plenty_ of room for trial-and-error before you have to delete any.
3) Turbine has implemented "character paths" which you can use for character generation and leveling-up; they cover several common builds for each class, and explain the role of each option. You can use this for none, one or all of your characters, and you are not locked in to it; at any level-up you can choose to go your own way.
There are fairly easy ways to revise or redo feats, enhancements, and bard/sorceror spell selection. Skill points and stat increases are permanent, but there are skill (+1 to +15 and more) and stat (+1 to +6 and more) boosting items readily available.
Alignment and last names are completely unchangeable. The biggest effect of alignment choice, so far, is to restrict the use of some weapons with certain kinds of alignment-based damage effects. Be aware, though, that there are a lot of lawful (evil) high-end monsters, not just chaotic ones; Lawful Good is an okay choice, but its not uber or all-purpose.
Also, as mentioned above, there are forum for both race and class discussions, where people post and debate their favorite builds, and a number of fan-made character planner programs that are widely used.
4) multi-class: you can train one or more levels in up to 3 classes total. Generally, your character level is the sum of all class levels. Also, there are a number of ways to use feats and enhancements to approximate the same result from a few different angles; _if_ you are willing to accept trade-offs, you can make your mage competent in repeating light crossbows, for instance, or blend wizard and rogue. As mentioned, only alignment prevents certain combos, like a barbarian/paladin.
5) Developers have said that the current prestige enhancements and the ones to be added shortly are likely to be added to in the future, as they figure out how to properly implement them.
The easiest source for basic character information (race, class, feats, enhancements, spells, named loot items and more) is the great DDO wiki http://ddo.enterwiki.net/page/Browse_portal . You can start some character planning here. As ntomlin suggests, a pure class might be your best first character.
The grind that lerincho aptly describes is very high level, and you can put a lot of time into that. There are lots of useful (non-raid) named loot items _below_ level 10 which you have a good chance of getting by doing the quest 3 or 4 times. If you don't have a more experienced character helping, you will probably want to do some quests three times anyway (once each normal, hard and elite difficulty), to maximize favor and gain helpful benefits like more backpack space, more bank space, and the ability to buy thirty minute buffs.
I think I've gone on long enough ... D'oh!
But now I know! (And knowing is half the battle XD)
