Since 2003, the game has changed A TON. Also depends on when you left in 2003; pre or post ToA. I'll assume post ToA, if not; I'll post again.
Class changes:
There are 5 new classes that joined the game after you left, you can find information on that.
Due to gear and additional power added to classes, most classes can easily chain kill yellow/oj/red and even purple mobs (when higher level).
Leveling:
Leveling takes little to no time now. 1-35 should only take you 3-4 hours (2-3 hours when you get playing again). This is done by getting to any major center (border keeps, capitals, SI main cities, the entrance of ToA)
35-40 are the pain levels now (I know before, 40-50 took the longest, not so much anymore); 40-50 you can mostly grind through questing... In every realm, you are able to run a quest from 40-50; which is pretty much a run here, kill 10 of these, port/run back, turn in for a ton of XP and 100some gold per pop. I suggest running this thing for a returning player, it gives you some starting money.
Additionally with the Darkness Rising Expansion, there are 10 additional champion levels. These give you additional hit points, basic abilities (slow casting, but highly useless spells, additional resists, minor heals, weak styles) and the ability to use Mythirians. Mythirians are generally useable from level 6-10; and can give very useful bonuses (you can look this up)
Gear/Templating:
Gear is a ton better, even RoG is better than old epic armour.
The big thing now in RvR (you know before, 50 + epic armour + some artifacts = good enough) is to have a template. Templating is a bit of a chore to pick up... once you pick it up, it gets easier but it will frustrate you to begin with (unless you find a template online, which I suggest... and then modify to fit your budget)
Gear can also make or break a fight. Having many /use toys and the top gear (recently ML10 gear got a huge revamp, and people using ML10 weapons have a general advantage over people that aren't... however it may cost a buck or two).
RvR:
RvR in this game is still its bread and butter. The three-tier faction system is still one of a kind.
There are several places to RvR: New frontiers (which also looks VERY different), Labyrinth, Darkness Falls and ML10.
The frontier has kind of become a pure zerg fest, not a huge deal if you're a classic type player that wants to do that. But very frustrating if you don;t like that kind of stuff...
The Labyrinth has a lower quality player base, RPs are easy and fast... a good place to be if you're just looking to jump in realm points quickly.
Darkness falls has pretty good rps and I've personally had some amazing fights (have also smashed RR12-11's that do nothing but gank level 15's) but it is less popular
ML10 can be fun; however there are generally raids occurring. IF you have a decent group you can pop in and kill 3-4-5 groups, however not everyone will be worth something.
RvR is still really fun, especially if you find some people you really like playing with.
Before 50 you can also frequent the Battlegrounds, where I suggested leveling from 1-35 to get some starting realm points (generally stops post the lvl 39 BG). The only thing about BGs that I don't suggest is... Molvik and Thidranki, the level 35-39 and 20-24 Bgs respectively have people who only play there. They are fully geared and kind of annoying to deal with when just passing through. So don't look too much into the BGs, especially on certain casters and heavy tanks. But you can get RR3 in Molvik... it's a good idea to get 2L4-2L8 before even thinking of moving on... Not required... but a good idea for a retuning player.
If you're not into soloing, and don't really know anyone... you'll most likely end up running with the BGs. Two realms has a few people that run massive numbers of players around the frontier taking keeps and relics. Hibernia really doesn't have a big name Zerg leader... So if you're starting back there... may be a bit of a pain. Most people who are just coming back, run with BG until RR4-6 (some never do anything but run with the BG, even after RR12) until they meet up with people they connect with then can branch out to do other things.
Anyways, any more questions. Just ask.
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Davolica, Fru Norsewoman Warlock. AMO Officer
Hayziis, Bantiarna Lurikeen Mentalist. GM Aes Dana
Macrina, Phoenix Knight Briton Cleric.
http://www.boandterri.net/winlok%20guide.pdf <-- My Warlock Guide